Because the marginal cost for a theatre to show a movie is virtually zero.
They can't just solve this by squeezing theaters, either. They're already struggling, if not barely surviving. If MoviePass simply tries to strong-arm them into accepting less than full price, without offering some other source of revenue in return, that's a lose-lose strategy for them. If they fail, they lose their ability to supply customers in that market with the service that they signed up for. If they succeed, they risk bankrupting the people who actually supply their customers in that market with the service that they signed up for, and so they still lose it.
Which of course hurts the ads/analytics side of their business. Not going to any movies means little or no data generation.
In every way they are better off with two million subscribers that go to 8 movies a year than a hundred and fifty thousand subscribers that go to 25 movies a year.
Likewise, the Webster Regal in Chicago is about $3 more per ticket than theater they dropped support for. But they don't carry all the limited/foreign releases that River East does.
The problem here is in the at the moment. MoviePass trains consumers to see the experience of going to the movies as effectively worth nothing. That is very bad for theaters, because when MoviePass goes away they will be left with a product---with small margins---that consumers see as incredibly expensive.
"Heads I win, tails you lose."
> The largest theater chain in the US instead described MoviePass as as an existential threat. “That price level is unsustainable and only sets up consumers for ultimate disappointment down the road if or when the product can no longer be fulfilled,” the company harrumphed.
(emphasis added)