Because people hate being coerced to do something. Stackoverflow didn't need to force people to do so. A Q&A site only purpose is to give you the best possible

Because people hate being coerced to do something. Stackoverflow didn't need to force people to do so. A Q&A site only purpose is to give you the best possible answer. FAST. Anything else degrades your experience. If quora breaks me out of the flow to register to their site ... they have cost me way more productivity than they could possibly bring back.