I honestly think that installing and using custom maps is EXTREMELY easy. Sure it could be all a 100% automated process in which you click a download button in your browser and a few seconds later you had it in the game, but that would be overdoing things if you'd ask me, question would be who would want and could make this happen? There's probably many things that could be worked on besides this, the installing and sharing of custom maps thats is not really tricky at all.
Especially if you compare with other games in general that exist out there. I just don't see any point of making something easy, easier, when there's other things that could be thought about.
To point at wolfies post, the only reason it wouldn't work would be because the one wanting a custom map really never used computers that much for other than browsing the internet, or the map itself would not be made correctly.