Cowboy Action Shooting has probably played a part. But i think the main reason is that money is chasing a supply that will not increase (no one is likely to discover a hidden arsenal full of old Lugers or Navy Colts) and can only decrease over time.

The prices of good-condition percussion Colts, Single Action Army's, Lugers, "broomhandle" Mausers, etc. - the stuff that collectors want the most - has been pushed beyond the reach of all but the wealthiest collectors.

So, attention among collectors of average means has shufted to things like pinfires, Bulldog revolvers, less-known manufacturers, etc. This is pushing up the price of these once relatively cheap items.