Because his entire story has been covered by KOTOR 1, 2, SWTOR and the wealth of EU material out there such as Tales of Jedi and Star Wars The Old Republic: Revan. A film would therefore just be a remake of existing material.
Why was this not suggested yet?
He has plenty of story behind him to make (in my own opinion) a VERY successful movie.
We also have to remember that Revan is not an iconic or even known character outside of the gaming world, he has no connections to the prequels or the original trilogy and will have no connections to the sequels. There is simply no place for him.
And personally I think 'The Old Republic' should remain in the gaming sphere and the EU, that is where it belongs. That is where it works. Also I think I've had my fill of Revan.