The fiscal cliff imposed a ticking time bomb on House Republicans. Unless they acted, taxes would shoot up for all Americans, who would immediately notice it in their paychecks. Gun control and immigration “reform” are fundamentally different because House Republicans can veto these things by simply refusing to vote on them. There are no immediate dire negative consequences for refusing to vote on these things. Sure, the Lying Liberal Media can bray; but they can only bray for so long.
Combine that with the looming fights over the debt ceiling, the spending sequester, and entitlement and tax reform, and these issues will be quickly moved to the back burner.