An official statement from Tumblr to us reads:
To our dismay, Twitter has restricted our users’ ability to "Find Twitter Friends" on Tumblr. Given our history of embracing their platform, this is especially upsetting. Our syndication feature is responsible for hundreds of millions of tweets, and we eagerly enabled Twitter Cards across 70 million blogs and 30 billion posts as one of Twitter's first partners. While we're delighted by the response to our integrations with Facebook and Gmail, we are truly disappointed by Twitter's decision.
Obviously, Twitter doesn't think what Tumblr is giving it is worth access to its follower/interest graph, which is the most valuable thing Twitter has. I took a whack at explaining this previously, but Dustin Curtis nails precisely why Twitter is so damned protective of it:
...most people do not create content in Twitter and therefore have no incentive to use Twitter outside of the value of its graph. Unlike replicating or using the Facebook graph externally, relocating the Twitter graph can have disastrous consequences for Twitter. Lots of celebrities use Tumblr, and if you can instantly relocate your Twitter graph into Tumblr, then what value does Twitter have, other than different content?
So the hammer falls.