The process requires filling out a bunch of forms, fingerprinting, photos, etc., then takes (most recently) 6 months to complete... And usually you have to pay 50% up front on the suppressor.
That complexity greatly reduces volume for the manufacturer. Another factor is that many people don't even realize that suppressors are legal to own (at least in some states). So they don't even try to buy one.
Heck, if suppressors could be purchased by just forking over $ without going through the tax stamp process, I'd probably own 5 or 6 of them vs. just the one...