They don't have the rights to The Silmarillion, and as long as Christopher Tolkien lives, the Tolkien estate won't give them up.
And The Silmarillion would need probably about five movies to really have enough time to tell all the most important stories.
Well he IS getting pretty old, it probably won't be too long. Dumb question but I don't know much about this legal stuff - how were the rights to the Hobbit acquired?
I bet they could do it in 3 - but they'd be long like the trilogy - and I'd agree it probably couldn't be done in any fewer.