I'm guessing either they were bound to abandon them eventually (illusion assets offer entry into game development with little to no monetary commitment) or they switched to daz3d or something similar so they would be able to sell their game on Steam. It's easy to abandon a game that's making you no money if you don't have any money sunk into it.
As for people who abandon games that are financially successful, well... you'll have to ask BlueCat.