The powder-type inks used in most Xerox machines 
(even very, very good ones) are volitle to fading 
under certain lighting conditions. . . The hand 
painted/inked lines are simply made of "better 
stuff", so to speak, and lack many of the 
weaknesses to which the Xeroxed lines are prone.

And by the way. . . I don't know when the 
Japanese switched over, but Xerox machines have 
been in use for animation since *well* before the 
1990's. I think the first US animation to use the 
Xerox machine was (and you'll all get a laugh out 
of this) Disney's 101 Dalmations. ^_^  (Yes, if 
you look at the movie close enough, you can find 
the repeating patterns of running dogs all on the 
same screen. ^_^)  At least that was Disney's 
first experimentation with using Xeroxing in 
animation, even if it wasn't the first in the US.

Many Sharp Smiles,
--Drac (who if she doesn't know something Disney
        has quite the adequate personal library to
        look it up! *laugh*)