This is an informal, unscientific, open-ended survey. I want to know what economists currently believe to be the answer to the title question. Please leave an answer in comments. If the phenomenon is multi-causal, feel free to elaborate, but I'm not looking for a survey of all the causes, just the ones that you believe to be correct.
I will not express an opinion myself, nor will I reply to anyone generous enough to leave their own. You may also reply pseudonymously, but since this is intended as a survey of economists, please indicate if you are one should you not give your name.