Its because they are excellent guns and they don't make them anymore. Simple as that. As near as I can tell, the average Colt collector ain't hurtin' for money and are more than willing to spend it for what they want.
If anything, I believe the 2020 Python actually drove the price of the older guns up. Many folks prefer the original over the remake.
Its the same with all the snake guns. I have a 1975 6" Python and 4" .38 and 6" .22 Diamondbacks. Any of which I could sell for a very nice profit. I always wanted a .45 Colt Anaconda. I'd have to sell one of my kidneys to buy one now.