Anon sites used to be hosted on LJ until the strikethrough of 2007, and then LJ was bought by a Russian company, and basically became the Wild West, so a lot of them migrated to DW.

DW/LJ were preferred bc not only can you be anonymous, but moderators can freeze/delete/screen comments to keep the communities SOMEWHAT sane.

I honestly cannot imagine running an anon community on tumblr. It's impossible to contain/organise the same way as it is here.