I would guess that you are borderline, although I wouldn't know if you
are on the WL/Reject or WL/Accept border. They probably just need more
time to review your file... time to flesh out the class a little more, then pass
it around the committee one more time before they decide. That's just my best guess, though.
It sucks that schools drag you along like that. I wish I had applied
earlier, oh well. Good luck!