The reasons:
-Ciminio's casting concept: The immigrants should be played by foreigners.
-Huppert was a star in France & well-respected in Europe.
-She was willing to be naked on camera, play a prostitute & have a rape scene.
American star actresses were not willing to do that.
-She had the potential to become an international star in the future.
I think Cimino's decision was sound, but a big American star
would have helped the film at the US box-office.
He was right about Huppert.
She IS now an international star & the best European actress.

I don't understand, what's the problem with her?

Well, you have the IMDB here, which makes weird that you say that her only US role was in that film, as Hal Hartley's Amateur was everything but non US.

Ehe was one of the few excellent things in the film.
Kiss
kiss,
Bang
bang

She is a wonderful actress, toplisted in France and rest of Europe. But alas not in the US (her only other US role after this was in IloveHuCkabees in 2008) mainly due to her performace in HG.
Again she is a great actress but not a real looker as Catherine Deneuve, Sophie Marceau or La Seydoux. In those days she hardly spoke a word of English and also had to learn to ride a horse (watch her galloping about always with a look of total fear on her face and I don't think she was acting then!).
Probably one of the many weird whims of Cimino to go for her.

I just watched Final Cut and what I found interesting is there was no discussion of why all the female big names passed. Here was an Oscar winning director, in a big epic film and yet the women passed. I think had UA addressed that they might have had a better film-namely that Ella's role was pretty poorly written.
That said I actually thought IH did as well as anyone could with the part.

They were scraping the bottom of the barrel after the big names (Fonda, Keaton, Field, etc.) had all passed on the role. Then the director became entranced with Huppert for some reason. I think insisting on her being cast became more about Cimino wanting to get his own way rather than any particular qualaties she may have brought to the role.
In other threads, posters suggested that Tuesday Weld or Margot Kidder would have been good as Ella, and these seem like better choices even if they were second-tier stars.
.

I would say because Michael Cimino was just like another 1970's film auteur, william Friedkin. Pompous and full of pretension, and nothing says pretension more than casting French actors for your films!
"Everyone else may be an a**hole, but I'm not!" - Harlan Ellison

Are you referring to the original poster? If not best to do some research
Vagabear
.

Why did you misspell her name?

The casting of this film was most likely a huge contributing factor to this films failure. Cimino had Clint Eastwood as a lead in 'Thunderbolt and Lightfoot' and Robert DeNiro in 'The Deer Hunter'2 big popular namesyet he chose to cast Kris Kristofferson as his male lead in 'Heaven's Gate'.
Kristofferson is an excellent character actor and support for other big nameshe is not a marquee light up. And who is Isabelle Huppert when she is not at home? Considering the limited appeal of the films subject matter and the large budget that was being allocated on thisCimino, who is a true auteurmade a big casting blunder, when what he needed were 2 big names to help attract. Who knows though if this would have made much of a difference.

at the time, top critically acclaimed english speaking actresses didn't get naked so often like today. especially in such careless way..

It's a theory, but nothing more Bach intimates at Cimino being smitten with her, but never mentions any knowledge of an actual relationship something he surely wouldn't refrain from doing, considering the tone of the piece.
If I can make a guess, I think he always wanted a foreign actress for the role, and his "promise" to cast an star actress was just lip-service (just like his promise to cast a star actor). Why Huppert? I don't know. Her career was finally taking off at this point. Cimino claims it was purely accidental: during casting, he happened to end up in front of a theater showing
The Lacemaker
and instantly knew he wanted to cast her. Of course, Cimino was never one to let the truth get in the way of a good story.

she was his girlfriend at the time. say no more.