That's probably because it didn't do well when it was released; it only grossed $10 million:
http://www.boxofficemojo.com/movies/?id=betteroffdead.htm
It has a cult following, and it has John Cusack, so that's why they're releasing it.
I'm really excited about that one, but whoever designed the cover "art" for it should be in Trouble with a capital "T" and that rhymes with "D" and that stands for "Dismissed."
I sincerely doubt Dumbo has been added to the Platinum/Diamond/whatever-jewel-they're-using-now Collection. A wonderful movie, yes, but it has not been out of print since 1980. I believe it's already on iTunes (although Aladdin is, too).