Because it's not sold as a pocket Linux computer, but as an "internet experience." I bought my first tablet because of Nokia's mobile expertise, and because it running Linux meant new and interesting apps would be plentiful. But I did expect *some* leadership in app design in the vein of S60 projects like Widsets, Share Online, Nokia Search or Podcasting from Nokia but instead got a beta of Video Center that seems to have been abandoned.
Ultimately, It's the core experience -- an 800px wide "full" web browser with Flash -- that keeps me using it, but honestly that core experience hasn't grown much since November 2005. And that core experience is owned by Nokia and theirs to change.
Things will improve for the community with Mer . . .