I don't know the history of *boorus either, but think of it this way:

World2ch and World4ch are text-only. There is a niche audience for these types of sites.
*chans have images and text. There is a niche audience for these types of sites.
*boorus are image-only (well, there are comments, but no one gives a shit about them). There is a niche audience for these types of sites.

If people will browse them, sites will exist.