I don't know about other people, but for me it's 2 or 3 things, I don't know about other people.
1. I am lazy and don't want to download an asset that I'd have to create a special page to attribute an author (especially since i have never created a special page before)
2. While I do love your stuff, very beautiful stuff. I Have no backgrounds to go with any setting that any of these characters would be in and have no idea where to find them other than AI art (which I don't want due to it's controversial nature) or commissioning someone to do them...(which I don't want to do for monetary and attribution reasons) while I could draw them I am not confident in my drawing ability being up to the character assets (they have to at least match in quality for me) (Manipulating photographs look weird when combined with anime/cartoon style art). So essentially on top of being lazy I am particular or picky.
3. While I do wish to write adult visual novels the characters don't need to be nude for it, only shirtless if male, so technically as long as I either have permission from the artist or they don't mention that they don't want it in adult stuff I can still do it. So technically if someone doesn't want to put your art in there as long as you have a basis for adult content through story-telling it's their problem not yours.
4. If I write a visual Novel I want to be able to charge money if I so choose and your permission on this point is vague: This is a non-commercial project. Does that mean that i can't use it for Commercial things are that you aren't charging for it, and putting it in the More information box makes me have to troll the internet to look up what the license is, and gives me permission to do.
5. I haven't currently got an idea for a story in my head for your assets. (Which isn't helped by not having backgrounds to put them against (Seriously the backgrounds is my biggest problem with pretty much any story)).
I suppose ease of use and not currently having something in mind to put the characters in is what this whole long comment is about.
I Love your assets they are very good.