It's all Marvel's fault.
Well, kind of.
See, there used to be a ton of comics distributors. There were two big ones -- Diamond and Capital City -- and a whole mess of smaller ones.
Then one year in the 1990s, Marvel decided to buy one of those smaller ones, Heroes World, and make it their exclusive distributor.
This was a disaster of epic proportions, as 1) Heroes World didn't have the infrastructure to suddenly become the only worldwide distributor of the largest comics company on the planet and 2) all the other distributors were suddenly denied their biggest customer.
In response, DC went exclusive with Diamond, thus costing all those other distributors their second biggest customer. Cap City and all the smaller distributors went out of business. Then Heroes World failed like a big, giant failing thing and was discontinued, and Diamond was literally the only distributor left standing.
That may be part of why they haven't been hit with any kind of monopoly penalty, because it really wasn't their doing, as such, they were just the only ones left after Marvel fucked up.