Well that's because artists put more effort into work-safe and ecchi drawings on pixiv (and other sites too).
The porn they post there is usually rush-job ideas that they have in their minds to spread out to the internet public as quick as possible. Because let's face it, there exist porn for
everything. But since there's a big amount of porn drawn without much effort put to them, you get to appreciate the more work-safe drawings more and admire your favorite characters in more diverse ways.
I don't mean that porn can't look good.
Our oekaki has lots of good ones (well, from our own perspective anyway, it might not be pleasant to other people, lol). But nowadays, porn in general doesn't look that pleasant (or maybe people are getting all soft lately), but for sure, when more detail are put into drawings, specially in the garment the characters wear and their beautiful looks, that makes art stand out more and more, and it's appreciated that way.
Like example
the avatar you have right now. So cute, adorable and beautiful~ ^o^