5. Because The Last One Was Awesome
Simple as that; because the last one was awesome. The idea is tried and tested and it evidently works. Movie studios know that there's an audience and they know that, if it's executed correctly, it could be just as big a hit as it was last time - if not bigger - second time around. It's a fun premise and last time the audience took to it when The Mask was a little-known 3 year old comic book. Now, The Mask has existed in comic form for 23 years, has already been a successful movie (albeit with a hideous sequel) and has had a good run as an animated television series. Essentially, now that we know that The Mask can carry a movie, there's no reason he can't do it again.