For one, it died out ages ago, another it was only ever popular with kids and anime fags like us
Transformers, anyone? :wink:
People don't stop being fans just because a series isn't on the air anymore, and I can name at least six people in my karate class alone who can go on at length about how awesome DBZ was.
Now I don't know whether this'll be a blockbuster hit, and I don't plan on placing any bets until I see--at the very least--a teaser trailer, but I do know this: people who grew up on Star Trek will flock to theaters for a new Star Trek movie, and by the same token anyone who watched and loved DBZ--or even people who didn't necessarily like it, like me--will pay through the nose to see this if the trailers impress them enough. The first impression is the key to success on opening night; after that, it's all up to the quality of the movie and the reviewers.
Japanese movies in general, yet alone a movie that will barely succeed in Japan, will never be popular in the rest of the world.
Obviously you've never heard of Akira.