What do you mean? Map making continues for COD4 to this day, what they did instead was completely wipe an entire forum (years) of discussion and tutorials and knowledge on how to do maps. Was there some Microsoft agreement to eliminate official support for mapmaking on PC (but still let the mapmaking continue?)
Compared to these days, the CoD DLC contract was a big deal. Who knows what Activision and Microsoft fully had in mind behind closed doors, but when MW2 was out, they wanted you to play on Xbox. This resulted in the shoddy PC port in general. And I don't know if you every played MW2 on PS3, but it's a night and day difference in quality between that and the 360 as well.
MW2 was gonna be big seller on PC as far as they knew. A big mod/map/server scene would kill the purpose of buying the DLC maps. They stamped it out real quick.
It was the time period really. CoD basically blew everything else out the water and Activision was looking at dollar signs. Obviously, the devs laxed over the matter for the next games, as the MW2 situation was a huge stain on the PC base.
Then a ton of people on PC quit and went to alterIWNet and that was that. RIP aIW and fourDeltaOne...