Maybe the critical difference is that if your account would have anything remotely adult at all, including text, or would repost anything from account that was flagged adult then your account would need to be flagged adult. And as a result a lot of accounts were adult flagged and a lot of users felt they had to log in and enable view-adult in order to not get left out. And then even if you have no particular interest in browsing the explicit content on tumblr you can't have many contacts before you start getting occasional reposts of it.
Regardless, being able to have a site which users can fully engage with without a steady stream of surprise crotch shots that they aren't interested in seems like a pretty reasonable goal. Banning the content outright seems like a really blunt way to get there, but maybe that was less damaging to the monetization strategy than making a lot of images click to load?