Krauthammer, in a creative fashion, believes that Obama's speech was moved inside because he couldn't fill the stadium:
Nate Cohn doubts this:
[T]he Obama campaign lost an opportunity. By all accounts, Obama's speech in Denver endowed Obama with an army of new volunteers and text messages to their friends and relatives. With Obama down slightly in most polls, a big speech to rejuvenate the enthusiasm of tens of thousands of North Carolina volunteers could have been extremely useful. Perhaps more than any other state, North Carolina is all about turnout. If switching venues cost the Obama campaign a fantastic organizing opportunity, the optics or weather risks must have been serious, at least in the Obama campaign's judgment.
Jonathan Cohn lists other reasons to believe the Obama campaign:
The first is that the decision will disappoint a lot of groups who were distributing tickets to members. The second is that the campaign will have to re-accommodate donors that paid for skyboxes. The third is that the campaign and administration officials I’ve seen today seem genuinely disappointed that Obama won’t get a chance to reprise his performance in Denver, when he spoke to the football stadium at Mile High.
Silver is more skeptical. Since the tickets were sold and there was a waiting list, I'm inclined to believe Chicago. But it strikes me that an outdoor stadium event simply wouldn't be right for an incumbent president in tough times. He needs intimacy with the average voter; he needs to explain; he needs to get away from all the celebrity crap that surrounds him. Inside is better – less hubristic, less dramatic, more real. A reader raises a good point:
I'm inclined to believe the campaign when they cite weather concerns as the reason for moving the speech, even if it doesn't end up raining. If there's a chance of rain (and as a North Carolinian who has seen the skies go from blue to black in a matter of minutes, "a chance" is often as good as a promise), people are going to wear ponchos and raincoats and carry umbrellas. It's one thing to get 65,000 people through security when the weather is nice – but add all the extra bulk and accessories that a rainy forecast necessitates, and you have a potential security nightmare.