* Tumblr was living from NSFW content and when they banned NSFW content Tumblr basically ceased to exist.
* Reddit is living from free moderation and when they banned 3rd party tools for moderation, because Reddit ones sucks, they might be going in same direction as Tumblr.
"Reblogs" are a bigger feature on Tumblr than twitter generally and the reply / threading structure is pretty unique, which has a tendency to create a lot of blogs that are essentially 100% reblogging other content.