Because she’s just basically popular in Maine:

Fascinating numbers for Olympia Snowe. Her approval rating with Democrats is 25 points higher than with Republicans- in fact her approval numbers with Democrats are better than they are for many of the Democratic Senators we’ve polled on across the country this year.

Like Ben Nelson on the Democratic side, she’s a GOP Senator in a state dominated by the other party. If the Republicans try to get rid of her via the primary, they’ll lose the seat, probably permanently.

(Hat-tip to Taegan Goddard’s Political Wire)