The movie was, in fact, held back for a bit. Part of this was a two-fold matter -- first, the movie wasn't exactly a wide-appeal movie, so selling it to audiences would be tricky. However, once Dreyfuss was a bigger star, that gave it its own obvious promo:
There was a notable "gasp!!" value due to a fairly major star being in a "gasp!" 'X' rated movie. Offhand, (excluding Midnight Cowboy, which is really PG-13 these days), only Marlon Brando's "Last Tango In Paris" was the only big-star 'X' movie.
The 'X' rating at that point wasn't completely associated with porn-only by then, but was well on its way -- that only took a few more years. One reason why they invented the "NC17" was to get past the stigma of an 'X' movie, which many theater chains would not carry at all, no matter the provenance.
NC17 got a little use for a while, but theaters were still dodgy about carrying them, so after a couple were released ("Henry and June", and "The Cook, The Thief...") they mostly became something to be avoided assiduously.
The 'solution' that has arisen is an R-rated cut for theatrical release and then an "unrated" version for video releases.
That's not to suggest nothing gets an NC-17, but it's usually smaller art-house films that settle for them, not major pictures playing to thousands of screens.