i guess one problem is:
people that really work with these genes are after the super forms, or the combined super forms.
so people have their ivory females, and then they breed a super stripe or a combo containing specter or a spark or a highway to them. in all these cases, you get super stripes or super stripe combos or highways or pumas, but when it comes to single-genes, only yellowbellys pop out of the eggs.
the market for super stripe is much bigger. i think noone is intentionally aiming to produce specters. you can easily find specters (with extra genes), when you search for super stripe. or super specter.