Bloodstained: Curse of the Moon is almost 5 years old. Inti Creates might still not want to release an explicitly DRM-free version, there could be extra legal issues, and/or they just don't care about GOG's small market share, but if you're interested in seeing Curse of the Moon or other games of theirs here, consider sending them a message:

It should only take a few minutes, I figure it doesn't hurt to try. Of course be polite in your message to them.
Press/Business Contact (I might be wrong but I think this is more relevant than the contact page below)
http://inticreates.com/contact/press-business-contact/

http://inticreates.com/contact/

Here are the two wishlist entries for the Curse of the Moon series:
https://www.gog.com/wishlist/games#search=curse%20of%20the%20mo
You could also suggest Inti Creates games people should vote for on the wishlist:
https://www.gog.com/wishlist/games

https://en.wikipedia.org/wiki/Inti_Creates#Games_developed_and_published

They could not have been clearer about what they think of DRM-free games. They explicitly said no to GOG because it's DRM-free.

Screw 'em.

No idea where the push for this came from, but I know that it is a colossal waste of time. Equivalency of pissing in the wind if you will.

I backed Bloodstained: Ritual of the Night and many backers watched as Inti Creates were silently removed from main game development and shifted to the bonus game Curse of the Moon instead. Nobody thought anything of it because we were promised everything to be drm-free. It wasn't until later that we found out that Inti Creates was forcing Curse of the Moon to be Steam only. Apparently they had been very vocal in the past about not releasing games without drm. When questioned about this in emails they further reaffirmed their massive hardons for drm. After that it became very clear as to why they were removed from Ritual of the Night.

That's fine if some people really feel like it'd be a total waste of time, but things and business decisions change over time. And again it only takes a few minutes so not much time wasted. ;)

If you (anyone reading this) want GOG releases (or DRM-free releases anywhere), I hope you're at least letting devs who might be more open to DRM-free releases know this.