Also a lot of his BS has been called, so his supporters aren't as enthusiastic as before. It also drew less than 1/4th the crowd of 2008. Of course it was expected to drop in number for his second inauguration, but that is awfully low.

I'm sure the weather factored into it but the bigger reasons are more obvious when you compare it to 2009: It was a reelection. People already know most of what he'll say because this was a "win" not a "victory." There were only 700,000 people as compared to the previous 1.1 million. Less people make less noise. There seemed to just generally be less pomp and circumstance for this event than before.