It's mostly because of the UV : the technic used to
transfer the powder to the plastic sheet is similar
to a laser printer and based on bright light.

You can prevent the fading by keeping a cel in place
where there is no UV. But if you keep your cel
in a room, there is always some UV even if it's not
under direct sunlight. A good part of those UV can
be filtered with anti-UV glass but not fully.

Cheers,
Yann Stettler