Why would they do this? First, because they can. Secondly, the jails would be, in their opinion, overwhelmed by real criminals.
On the other hand, the government can put forth some recommendations for gun control that does, I suppose, look to some (perhaps most) in the public like they (the politicians) are actually doing something – even though the laws proposed would be largely ineffective – and they know it. But, the hard cold fact is that the government considers the general public to be pretty darn ignorant; and that legalese-laced platitudes will make said public temporarily feel good; but, more importantly, perpetuate the careers of many current politicians.
Furthermore, what's the big deal about a few more armed x-felons running around terrorizing the public? At least they aren't clogging-up our prison system (True's sarcasm).