Honestly, from my 2+ year experience in ow cheating, I still am not sure why. But I would say it could be because of the anticheat, like valorant is more expensive than overwatch, while csgo cheats are like $20 a month at max. Overwatch cheats also have a lot more features other than just simple aimbot. Critical Hit and Edenity, for example, have features like dva auto-eat, hook prediction, and sleep prediction.
So two possible reasons could be because of the anticheat and features they have. It could also simply be due to fact that people are greedy and the devs agreed to have that price set around that range due to a dying playerbase.