Since we're being snarky- perhaps read the articles you toss out? It specifically says that a major cause was the merging of their sales/marketing/ads teams and

Since we're being snarky- perhaps read the articles you toss out? It specifically says that a major cause was the merging of their sales/marketing/ads teams and installing middle management (between Marissa and Karp) who knew little-to-nothing about Tumblr, much less how to run it. It's not like Yahoo bought them out to turn them away from mobile. Or even to help Yahoo become more mobilized. They wanted to monetize Tumblr and gain from it's younger audience.