Well what you said basically.
The OSI series is a very limited printed manga and always completely sold out. At second-hand stores they disappear as soon as a copy appears and at auctions they always get sold for at least twice their original price.
You might not like them, but that's completely irrelevant.
Naturally it's very understandable that nobody would want to share their copy after all the trouble to go through acquiring one.
But interestingly enough not even the very easily obtainable digital editions aren't getting shared as well.
If that's not a statement then I don't know.
Tagame however, well the magazine he draws for is getting shared at some website posting gay magazines. So of course the manga appear sooner or later here because all people have to do is grab them there for free and repost them here.
That's I believe all there is to it.
Sure Tagame comics are great too, but not as rare and exclusive as Fujimoto.