I don’t think that’s true at all - in fact, I think that might be a point to SUPPORT the choice of those to license.
JAST does not seem entirely opposed to the concept of negotiating to acquire an existing fan translation of a game, which would drastically cut the costs and time required to release a title. Furthermore, I highly doubt the market is anywhere near exhausted. I think it’s always been the case that import eroges are more difficult to buy than others unless they’re available for download (which neither of these are), and Saya no Uta is a relatively old (6 years or so?) title.