You would have to ask them, but I would say the answer is "because they could".
Creative people are always looking to express themselves in different and new ways.
Edit:
And it was a gimmick.....Disney animated movies had been not as successful at that time as they had been in the past.
Of course I think that has less to do with the fact that it's cell animation, and more because they started writing terrible stories!
The movies coming from Japan from directors like Hayao Miyazaki are pretty much all cell animation and those movies are both beautiful and have wonderful stories!
Look for "Princess Mononoke", "Spirited Away", "Howl's Moving Castle" and out in theaters on the 14 of August "Ponyo".
Bookmarks