onlyaw said:
Pixiv is great, but the English tags are shit. Sites like this make the search for art easier, while giving credit, unlike most other places on the web.
So why do some artists not desire the benefit of exposure to an audience who enjoys their genre of work?
Because the voices in their head tell them so.
To give a more serious example, being able to retract an artwork can be important to an artist. Sure, once an image has been posted and someone else has saved it to their hard drive or device you can't truly eradicate it, but that's very different from an image being continuously hosted on a site you don't have any control over.
And as another example, a Japanese artist may not have any interest in their acclaim with an English-speaking community, especially if they're trying to make any amount of money from doujin sales. It's quite difficult for Americans (or indeed anyone outside of Japan) to buy doujin even if we want to - or put another way, it doesn't make economic sense for a small-scale doujin author to publish their work anywhere where non-Japanese fans could readily buy it. (The discussion on post #2428487 goes into some detail on the challenges involved in trying to buy doujin if you don't have a Japanese mailing address and can't get to Comiket and the like.)
Still, in a way, their reasons for not wanting their stuff here are irrelevant. Unless they declared their work public domain, they retain the copyright and have the absolute legal right to demand it be withdrawn from anywhere they haven't licensed it to, and need no other reason beyond "because I say so". (International copyright law can be confusing in the gray areas, but this isn't one of the gray areas.)