Supply and Demand, my friend, Supply & Demand.
Lots of them available. That's the supply side.
Truly unpopular calibers. I personally like the CZ and its cartridge a lot, the Tokarev round is especially impressive, but even though I've been reloading for over thirty years, I'll probably get rid of the gun once the supply of cheap surplus ammo dries up. The Nagant round is truly a weird duck, and although the gun can be converted , or the ammo reproduced, most people are simply not going to go to the trouble or expense for that particular gun.
I don't think the quality is the issue, both are fairly well made guns, especially considering the time and place they were made. But you have to be peculiar beast, like me, to want one.
Compare that to the P-38's that were sold as surplus a few years back. Older design than the CZ, but a lot more familiar to most, especially my generation who remember them from WW2, and in 9mm to boot. They went like hot cakes.
I can still pick one or more versions of the Mosin-Nagant rifles here in Phoenix for well under a hundred dollars. The chain that sells them says sales are down, and they don't know why.
Could it be (just guessing, here) that I usta could buy a hundred rounds for less than ten bucks, and I now pay that much for twenty rounds of the same ammo? (But gee, it now comes in a pretty cardboard box.)
The real surprise to me in all this is that I can still pick up a 98 mauser pretty cheap. But I suspect that won't last much longer, as I here the supply is drying up.