Why? Because she gives conservatives a bad image
Nothing bad about Bachmann's image, other than that hairdo she had last night - which was a mistake. The long hair she usually lets down gives her more sex appeal to the average heterosexual male voter.
She's got no skeletons of any significance in her closet, and she was seeking to draw a contrast with Gingrich on ethics and integrity, and with Paul on foreign policy and national security. She is a middle class woman of faith who seems to "walk the walk." And she's not a "career politician," having entered electoral politics at age 44.
In her position as underdog, last night was her last opportunity to make big splash in Iowa, and she succeeded. Like her or not, at least people were talking about her. It sure beats being neglected, as she pretty much had been in recent weeks and months.