There was no other choice. This seems crazy, but think about Ritter's options. He could have gone with Joan Fitz-Gerald, Peter Groff, or Andrew Romanoff, all of whom have good reputations from their work in the state legislature. However, it wouldn't be surprising if there was some bad blood between the governor and legislative leaders. Interbranch rivalries are real. Ritter also could have gone with Rep. Salazar or Rep. Perlmutter, both of whom have shown an ability to win in moderate districts. But that would expose a congressional district to a special election which the Republicans might well win at this point. He could have gone with Rep. DeGette, but she's widely known as the state's most liberal member of Congress and would likely lose in 2010. And maybe Hickenlooper didn't want the job (see above). So who's left? Bennet.
The various partisans for the other candidates were all convinced that their personal preference was the best and only logical choice. In reality all of the candidates faced significant hurdles.
It's no secret that the Governor and his staff don't much care for Joan Fitz-Gerald and her political style, I doubt she was ever seriously considered. There was tension between Speaker Romanoff and the govs office as well, no matter how good of a public face they put on. Romanoff was very anxious to get some movement on education reform at the start of the 2007 session and the governor dragged his feet for months and months. When he did jump on board with the Speaker's agenda he took it over and slowed the process down by creating another commission (the P-20 Council) that took many more weeks to create. The governor essentially sunk Romanoff's legacy project. I can't imagine that there's not lingering tension in that relationship.
Seth's analysis of the political calculations that wrecked Reps. Salazar and Perlmutter's chances is, I think, probably spot on. Either one of those seats would probably have flipped to Republicans.
I think another option that Seth proffers was also at work,
Goo goo. Ritter just likes the idea of good government and thinks that Bennet, regardless of his prospects for a 2010 election, was the best choice for senator. It's hard to believe that someone would rise to the rank of governor with such a sincere (which, in political science, is a synonym for näive) world view...
On some level Ritter really is that sincere. What you see in press conferences or on the stump is the real man. He's just not a cynical guy and he sincerely cares about being a good steward of the office for all Coloradans. He doesn't always succeed, no one is perfect obviously, but I can certainly see where he would end up choosing Bennet after looking at the flawed roster of candidates that he had.