Why wont Democrats brag about the Healthcare mandate during the Midterms?

Mainly because of public backlash. The backlash itself is due to a campaign of disinformation against it that basically said that people are literally going to go to jail for not abiding by this mandate, when in reality the mandate does nothing more than ensure that any money that's sent out to people to buy health insurance is either used for that purpose or returned to the government. Besides that, though, you seem to be focusing overmuch on the Democrats here. The Republicans mentioned that their basic strategy is to attack the Democrats and not present any policy decisions they would want implemented. Both sides are doing this for political reasons.