I read the comments on the Kotaku article that linked here and there were several wondering why we chose to do a unified style for the game instead of just mixing the styles like Kingdom Hearts did.
The answer is really simple. Pixar and Disney said we couldn't mix their characters as is. John Lasseter told us that the only way we could get approval is to make them all from the same toy line with a unifying aesthetic. This is also why the "Disney stole from Skylanders" thing is silly: we were always doing toys or Infinity would never have left the drawing table for approval reasons. Getting everyone on the same page with the characters' looks and the Infinity style was nothing short of a miracle, not to mention that it was the first time any of us at Avalanche have had to design physical toys (which also created huge obstacles for the style).
I don't know what strings Kingdom Hearts pulled to get all the characters together in their original styles, but that option was off the table for us. And frankly I'm glad. Maybe it's the persnickety artist in me, but even though I like Kingdom Hearts, I find the complete shifts in style jarring.