Something about live action seems to make it the pinnacle of adaptation.
Most things seem to get adapted to live action and rarely vice versa.
For example anime usually comes first then a live action, not an anime based off a live action film.
Video games are sometimes adapted to film. In reverse games based off movies usually suck.
Animation also appeals to a much narrower crowd.
Video games that get made into movies tend to suck, and movie-based video games usually suck as well. So far the crossing over rarely works in either direction (with some exceptions).
I think Highlander was adapted to an anime. And I heard recently that Supernatural is getting an anime adaptation as well.
Plus there are tons of movies that have been adapted into American Cartoons (though not always clearly related to the original film):
Ghost Busters
Clerks
Godzilla
The Mummy
The Mighty Ducks
Robocop
Star Wars (a couple of 'em)
Teen Wolf
Police Academy
Beetlejuice
Bill & Ted's Excellent Adventures
The Wizard of Oz
Back to the Future
Dumb and Dumber
The Mask
Men in Black
King Kong
among others.
(sorry, once I got onto Wikipedia I got a little carried away.
)