It's both, plus the legality.
There are legal ones around from years back, and then I don't know maybe some of the histros are now slyvatica or vise/versa... I'm not up on all that as much as some other people, but basically they're rare, they're pricey and some are of questionable origin. Also given the high demand and low numbers, most of the buy/sell/trading of these goes on in private. There just isn't a need to advertise for most of the people who have them and are willing to part with them.
The only recent legal imports of similar frogs I can think of off hand are the understory Slyvatica's
Oophaga sylvatica | Understory Enterprises
And hopefully Tesoros will get to do their imports of histrionica...
Occasionally people import them from the EU I think. Maybe other people know of more that have come in
The bullseye morphs are mostly what I like. Basically I like red, blue, black, white, and green frogs. If they're brown, yellow, orange etc.. I'm often not a fan. So I don't keep well informed on histrionica since most of the ones I like aren't here yet... at least not within my reach, but I hope that info helps some