Because gun purchases are a high chargeback risk; this is explicitly the reason that PayPal gives. I had a colleague of mine have a debit card stolen from them and then used to try and purchase a handgun. Given the criminal utility of a firearm purchased under a fake name with a stolen card, I doubt this is a rare problem.

(PayPal also cannot be used for cigarettes or porn, so they have a policy of avoiding what they see as high risk items, including items regularly handled by other card professors)

On the flip side Amazon will sell all kinds of firearm related accessories, just not the firearm itself or ammunition, presumably because they're not actually a FFL and can't handle the legal requirements.