I think it was because he has been hedgeing his bets a little too much, especially in a field this large and diverse. I definitely think he will come back from this (primary politics is notably unpredictable), but he needs to bet a little more on his original libertarian values. The Neocons will never like him, he has to make them like him by winning.
I agree with this reasoning. I would also note that by doing so, Paul lost one of his advantages; the advantage of polling stronger against Hillary Clinton than any other Republican.
Aside from the shift toward being a bit of a conventional Republican, and away from being a libertarian Republican, Paul suffers from a rather unimaginative campaign. This may be OK in the longer run, but Paul does need jolts NOW (or, more specifically, as Iowa and New Hampshire come around).