It would take a combination of accumulated goodwill, unity, a significant legislative majority to overcome the filibuster, and a perceived mandate from the people (particularly in certain swing states) to survive the blowback that would follow pushing gun control on the level of another AR-15 ban through — plus a lack of other competing legislative priorities (e.g. health care reform) that could suffer as a result of the gun control push.

This lack of overwhelming political firepower and clear focus on guns and guns alone is why they haven't acted, not any kind of goodwill or sudden appreciation for the Constitution. They haven't banned common firearms yet because they can't, not because they don't want to.