The issue was, they pretty much all agreed on the major issues. Halvorsen has a RECORD of speaking out on conceal and carry and spruiking her A rating from the NRA. That's the thing about records, they tend to stick to you.
Gun violence is especially an issue in that part of Chicago, so it became the dividing line. Halvorsen coming out for Universal background checks doesn't really address the issues of conceal and carry nor talking about her A rating as a badge of honour.
It's her record and if Bloomberg et al didn't have anything to use against her... then... yeah.
Plus, this is a special election in an off-year... you aren't going to get normal GE voters out for that. You get the passionate ones...
In what way does a member of Congress get to affect concealed carry laws?