The "omg you gotta play this" factor of it helped. Eversion was almost like a prank to play on your friends since it had a "surprise" to it. While you'd think that would hamper word of mouth, on a small scale it really encourages people to directly inflict it on their friends instead of just talking about it.
This seems to be the pattern with a lot of the cult indies, like Yume Nikki and IWBTG. Neither of those games is particularly polished (nor is Eversion for that matter) but they make up for it by having something striking about them that generates word of mouth.
I haven't looked into it very deeply, but it seems to me that the audience for games like Eversion is a bit different than the standard "indie games" scene. Places like TIGsource seem to gravitate toward amateur-produced but still slick looking games like Cave Story or Braid, or arthouse bullshit like whatever Cactus shat out today. These people want to be "real" developers, so the appeal seems to be "hey look how beautiful/important a game can be, even though only one or two people made it". And they still insist that Seiklus was good, so fuck 'em.
On the other hand, stuff like Jinsei Owata, IWBTG and Eversion seems to get picked up by places like Youtube and 4chan, where the audience is less concerned with some kind of amateur-hour circle-jerk and more interested in "hey check out this weird thing I found on the internet".
The problem of course, is that the latter audience is probably much less likely to actually BUY anything. I don't know if a game like Eversion can really succeed in the marketplace, because the things that attract people to it are paradoxically the same things that might be a barrier to people actually wanting to pay for it.