Blame TV. Animation in the US began as an adult medium, interstitial films that ran between the A and B halves of double-feature presentations at theaters as far back as the 1920s. Suggestive themes were commonplace—plenty of early Betty Boop material was explicitly sexualized; Red Hot Riding Hood's original ending was censored for reasons of implied beastiality.
The emergence of broadcast television and its place at the time in the American home meant that shows needed to be suitable for the entire family. The development of the season structure and the need for 10, 20 or more episodes put immense pressure on labor-intensive animation production, leading to the birth of the limited animation techniques. From Hanna-Barbera productions in which only one part of a character's body was being animated while the rest of the frame was held still—often quite obviously, due to slight color mismatches—to Cambria Studio cartoons like Space Angel which superimposed live action mouths over completely static images, the plummeting production values and increasingly childish plotlines rapidly alienated adult audiences.
At the same time, the rise of television hastened the decline of the double-feature: with more moving picture content to watch at home, the cinema became a slightly less special place (a trend that continues today), and with the double-feature gone, the animated interstitial all but disappeared.
(Pixar has the tradition of airing shorts before its main features, but the main features are animated, anyway, and aimed primarily at younger audiences, even as more complex themes for adult viewers may be woven into the narrative.)
And so US animation came to be dominated by children's fare by the early 1960s, leading to a public that deemed it intrinsically so. Animation morphed from medium to genre, and the association has stuck for over 50 years.
No discussion of adult animation in the US is complete without a mention of Ralph Bakshi. Fritz the Cat, adapted from the R. Crumb comic strip, was the first animated feature to receive an X rating from the Motion Picture Association of America. Highly successful, he followed it up with Heavy Traffic, Coonskin, Hey Good Lookin', Wizards, the 1978 animated adaptation of The Lord of the Rings, American Pop and Fire and Ice. He also directed Cool World, which, like Hey Good Lookin', featured direct interaction of live action and animated characters, in the manner now most strongly identified with Richard Williams' Who Framed Roger Rabbit?.
Bakshi's films are nearly all cult classics, and the man himself is an industry legend, but they weren't all commercially or critically successful in their time. Most crucially, they didn't materially alter the perception of animation as a children's medium. In fact, Bakshi was often regarded as a maker of "dirty Disney flicks," a perception likely stemming from his use of anthropomorphic animals in sexualized situations in Fritz.
The preoccupation with sex in Bakshi's early feature work actually hurts the case for animation as an adult medium with its rather juvenile prurience. While it may have rebelled against the idea that the artform was purely for children, it didn't tackle mature themes of the heart, mind or soul the way live action did. His later films did, but by then the audience had largely been lost.
Glossing over the odd outlier, that brings us to today. Not only does the perception that animation is for children persist, but animation is often treated as a genre, not a medium. Films with no similarity in plot, characterization or narrative technique are lumped together because the images on screen are produced by drawing or (exclusively) by computers. While this is technically incorrect, a cursory glance at recent domestic animated theatrical releases reveals film after film constructed to appeal to children. Sure, there may be meditations on mortality in Up, or commentary on wastefulness and gluttony in Wall-E, but the character designs, casts and overall narrative arc are designed with children in mind.
The fact that this is not the case in Europe or Japan is interesting. The difference seems to stem from the much greater commercial pressure on American productions, and the relative obscurity or inaccessibility of the arthouse to the masses. Over time, that effect has compounded to relegate even those adult animated features that do see theatrical release in the US to limited screens and showings: The Triplets of Bellville and The Illusionist, Persepolis, The Secret of Kells, Mary and Max, The Rabbi's Cat, Chico & Rita—none of these were US productions, and none of them saw broad release stateside. The closest thing to an exception is probably Rango; Battle for Terra flopped, as did Delgo.
Perhaps the relative affluence of postwar USA in comparison to nearly all of a rebuilding Europe or Japan factored as well, with the television working its way into the contemporary fabric faster in America. Modern Manga is said to have emerged in 1950s Japan, occupied by the US and influenced by the comic books G.I.s took with them and had imported, and by images from US television. It could be that Manga occupied the place in Japanese popular culture that television was assuming in the US, and, due to the relatively constant effort required to produce any two comics of similar visual complexity, a variety of genres for audiences of all ages emerged.
This hypothesis is appealing because Americans today, unaccustomed to thinking of animation as anything other than a children's medium, have come to accept the "graphic novel" as a source of complex narratives—even though they prefer to see the live action film adaptations thereof. It would seem that in a crucial moment of openness to germination, Americans were presented with a production and distribution medium that infantilized the material and came to consider medium and message an identity, intrinsically juvenile.
So now what? US TV is in an extremely interesting place now: its reach such that complex, dark, violent, sexual, uninhibited, profound, introspective, absurdist but above all quality shows abound, their critical acclaim, commercial success and force of sheer number seemingly raising the tide and elevating the medium as a whole. Television has usurped film, the small screen affording the luxury of time and space to plumb depths of character and plot that silver screen running times simply do not.
Crucially, television has been disintermediated. Sure, what gets broadcast still has to find network approval and survive the gauntlet of upfronts, pilots, notes and pickups, but what doesn't get broadcast—or in some cases is never considered for broadcast—can find huge audiences and tremendous success online, from YouTube and Vimeo to Netflix and Amazon. So successful has on-demand streaming been that it has cracked the rigid, five-decade-old structure of TV shows.
Where an aspiring show once had to potentially offer multiples of 12 or 13 episodes in a season, possibly supplimented by a few repeats, we now see more and more mini-series with only as many episodes as required to tell the story. Where episodes were once released weekly, with a possible mid-week re-air; then made available online a day, then an hour later; we now see Netflix making entire seasons of shows available at once. Where live sports was guaranteed as the last bastion against truly a la carte programming, the WWE Network just made all of its programming available online or on supported devices, for $10 a month.
There has never been a better opportunity for a growth in adult animation in the US. The audiences are hungry, eager to explore worthwhile niches. They are seeking substantial material, with fresh presentation. They are potentially more receptive than they have ever been. Television, the medium which inadvertently consigned animation in America to a "children-only" ghetto, can be exploited to lift it into spaces previously reserved only for Serious Things.
I, for one, can't wait!