The word in most cases is tradition. Back in the days of #2 cotton,
we painted the fabric with silver dope and put white over that. It
made it last longer, so, we still paint corporate jets white - go
figure.

I don't know about the newer composite aircraft, but most fiberglass
gliders are restricted by their type certificates on how much and
where you can put other colors so you don't become Icarus in the
bright summer sun.