Because pulp-adventure movies are great and there should be more of them. It shouldn't be hard to get a good blockbuster out of the Uncharted games. The characters are well-defined, the stories are interesting even if a bit familiar, the locations are beautiful, and the action set-pieces are pretty much already in place.
The Assassin's Creed games are a convoluted, jumbled mess of a story. The original premise is interesting and could make a good, interesting sci-fi, blockbuster, but if not done well... It will be a disaster. Considering the acting talent signed on to the project, I'm optimistic. But it'll be way harder to squeeze a successful blockbuster out of the Assassin's Creed games than it would be with the Uncharted games. If Sony had their shit together, Uncharted would make bank.