Democrats hand out committee chairmanships by a blind seniority rule. Republicans do not. Chairman need to rotate out of their positions after fixed terms, which then gives the caucus as a whole input over who takes over next. Consequently, the Senate leadership has some meaningful leverage over Republican Senators—even Senators from liberal states. If they’re really determined to make Snowe (and Collins) vote “no,” they have tools at their disposal to make that happen. By contrast, the Democratic leadership heads into tough fights basically disarmed with no real tools of discipline and leverage at their disposal beyond the vague risk of a primary challenge.The incentives here extend well beyond people who are already committee chairs. If they want to chair committees someday, Republicans need to be team players and do things that advance the interests of their party as a whole. You can't get in the way of legislation that the bulk of your party supports, and you have to act in ways that will retain the approval of mainstream conservatives in your party, or else you'll never get a gavel.
Meanwhile, if Democrats want to chair a committee, they just have to survive long enough. If you think that obstructing progressive legislation and practicing diabolical Liebermania on cable news shows will help you get a moderate reputation and keep your seat, there's really nothing your colleagues can do about it.