Marco Arment:

Google Reader is just the latest casualty of the war that Facebook started, seemingly accidentally: the battle to own everything.5 While Google did technically “own” Reader and could make some use of the huge amount of news and attention data flowing through it, it conflicted with their far more important Google+ strategy: they need everyone reading and sharing everything through Google+ so they can compete with Facebook for ad-targeting data, ad dollars, growth, and relevance.

Bingo!