I'm sure that the economy, unemployment, and crime rate have something to do with it also. However, I'm not sure it's the only driving force behind the change to handguns.
With the urban population surpassing the rural population, there's more people in need of handguns than long guns. I think that is the key.
In addition to that, there's the fact that we need to hunt less now than 50 years ago. We're allowed to hunt less now. Thus, personal protection is the focus, not hunting. This also means more handgun purchases than rifles/shotguns.