Speculation though. I suspect the SE's are finding there's a lot of noise in the signal. And they can't count on the signal - way too easily biased.
Links as something that can be measured aren't going away AND they're a broad overview of the web. Measurements of user behavior can go away. Chrome can disappear from use. cookies can't be trusted. Much of this is transitory in nature - they jump over to using the Google PR toolbar as a reliable indicator and people upgrade to a browser that doesn't have it, they're toast. Plus, if they used this stuff in any noticeable way I'm pretty sure it's way easier to game than links.
Same problem with using social media. Google's not stupid enough to substantially change their entire algo to depend on data from a single website like twitter, particularly when they don't own twitter. If twitter feeds start actually counting substantially in the algo, and MS buys twitter, how's that working for Google's search results?