Ancient-Red-Dragon: This is my theory:
GOG "curation" rejected Senran Kagura games years ago in the past, at the time when Marvelous first submitted them to GOG, on the grounds that they violated GOG's previously-held (up until a few months ago) Puritan-minded censorship standards.
A few months ago when GOG finally abandoned their Puritan-minded censorship standards, Marvelous either is/was:
a) unaware of GOG's abandonment of those standards or
b) Marvelous doesn't care to go through the bother of re-doing the work of re-submitting the same games to GOG which GOG had already previously-rejected for ludicrous reasons.
Of course, I could be wrong about that, since it's just a speculative theory. But on the other hand, it's a plausible explanation.
In any event, someone should contact XSEED directly and explain that GOG has recently, at least somewhat, abandoned their long-held censorship-minded Puritan standards, and see if they'd be willing to (re)submit those games to GOG "curation."
Yes, now that GOG has backtracked we can enjoy our degenerate animu games!. Not like their Based Puritan standards ever applied to titles like "Soldier of Fortune", which are arguably worse.