Reasons I can think of....

Gelkote is not transparent like varnish

Gelkote does not cure properly over epoxy

Gelkote requires spray application, wet sanding & polishing to have gloss if you don't use a mold.

Gelkote is heavy

Opaque finishs like gelkote & paint require a higher degree of fairing to look good.

Gelkote has only fair to midland UV resistance, but can be polished & waxed to improve shine when it oxidizes. Not a big problem unless you leave it out in the sun all the time.