A lot of reasons. It isn't mass produced like LSD or other research chemicals. It's kinda why you don't find many people selling mushrooms either - too much work and not enough profit.
Have you seen what DMT costs a gram? Mescaline is going to cost at least double that - probably more like 5 to 10 times. Most dealers arn't going to find many people willing to pay hundreds of bucks for a trip. They will find plenty willing to pay 2 bucks for some acid. That's the kind of price differential we're talking. You can ignore all the kids who say "Dude, I got me a grip of mescaline for 20 bucks".
You have a point there, and if you said that some of the mescaline going around is fake I'd agree with you. But some vendors focus on providing the rarer things to people that want them and aren't stupid enough to be fooled by a random chemical being sold as mescaline. I really don't think that all of those are scams. Now of course unless I buy some and have it tested this is all speculation.....