Democrat John Hickenlooper is really popular. For whatever reason (we could name many reasons, but this isn't the space for that), voters across Colorado really like the Denver Mayor. Elections are often popularity contests, because most voters do not educate themselves enough to know much of a difference between the candidates. It's more important for voters to like you than to agree with you on policy. Look at the 2004 Presidential election: A lot of voters knew that George W. Bush had not done a very good job in his first term, but they just didn't like Democrat John Kerry (and who could blame them?)That's a rather crude and cruel assessment of the American voter. It's also, as far as we can tell, pretty wrong. Most voters, political scientists have found, vote consistently with one party or the other, so they're not being swayed by likability. How do the rest vote? To an astonishing degree, they cast votes based on substance: the state of the economy, the presence or absence of war, and the relative moderation or extremism of the candidates.
Why was Bush re-elected? The economy was growing -- not hugely, but it wasn't in recession either. The war with Iraq wasn't a great success, but it wasn't a national nightmare yet, either. To the extent voters were retrospectively judging Bush's first term, they gave him a low, but passing, grade. Hence his receiving the lowest reelection margin in presidential history.
Why is Hickenlooper popular? Yes, he's got a great personal style, but part of it has to be that Denver has done quite well under his leadership. The city remains a very attractive place to live, and parts of it that once weren't that interesting (e.g.: downtown) are now thriving. That's not to say that all of this is due to choices made by Hickenlooper as mayor, but he is benefiting from actual substantive evaluations of life in the Denver metro area. If Denver had started to look more like Detroit in the last few years, I guarantee Hickenlooper would not be perceived as so charming and lovable.