Errr, we're wealthier, manufactured items are a lot cheaper, and it's a lot easier to keep a gun in working condition now that we're using smokeless powder and non-corrosive primers. Crime's a lot higher, I gather, so city folk have a greater need to be armed.

I don't have figures, and most of academia was delighted to celebrate a fraud who claimed very very few owned guns in colonial times (http://en.wikipedia.org/wiki/Arming_America) so data is probably hard to come by, but I wouldn't presume that at all.