They should have thought of a good plot before making the game if they wanted to make a movie later, and the game already did all the cheap movie scare tricks so what's left? Plus Isaac would gave to talk...
The game was so boring and scripted, I can only imagine all a movie would do is emphasize how bad the game was, and get compared to event horizon, Pandora, and a dozen other mediocre movies no one remembers the names of without googling.