I like it because it's not personal. I have a hard time finding (and keeping) friends on the internet, and having a Live journal account without anybody to comment on your entries is pretty frustrating, but it doesn't matter on tumblr.

Furthermore, my current fandoms don't really have a big lj-presence.

They do have a big tumblr-presence though.

Also, I prefer fanarts to fanfic for fandoms I'm just temporarily in.