People spending $ on games would rather just buy the item directly rather than buy a third party currency to bid on the item for weeks, work out a time to get on to trade and then get scammed by some 36 year old from Quebec named HorseDick420 whose most creative act to date was putting xXx in front of his username. It’s just a lot of extra time and effort to accomplish the same thing, and it’s much, much easier to avoid this site altogether if you’re spending money on games.
There are other reasons, of course. But the only market where it was ever cheaper to get things on jsp was for d2, which nobody really plays anymore.