There's a lot of credence to the theory that the film was in the wrong place at the wrong time. Studios wanted to wrest control back from filmmakers. People were sick of dismal 70's films. And the studio (Cimino, really) made a critical error by withdrawing the film from release and cutting it down to two hours, which was an admission that the first critics who reviewed it were right.
I still think there's a great film somewhere in between the 219 min cut and the 149 min cut. One is too long, the other is too short. It's difficult to see the short version, but if you can find it, it's fascinating to see how they gutted (and in some instance, improved) the film. Like DUNE, it's a film that will always seem unfinished.
Please read FINAL CUT by Steven Bach, who was an executive at UA during the making of HEAVEN'S GATE. It's a terrific book. Combined with the Heaven`s Gate documentary, you'll get a pretty good picture of what happened.