Give me the reason riot doesn't sell map skins

The same reason they don't sell custom announcers. They'd prefer to make money in less fun ways :(

Probably cause they have a hard time implementing the coding to change maps. Just take a look at Aram. Apparently, the Butcher's Bridge is not simply a map change, but a completely different game mode. Howling Abyss and Butcher's Bridge are two different game modes. I'm guessing that each map or mode requires an update, which i why new or old maps are rarely implemented in game.

In short, riot doesn't have the coding to change the maps. All maps are their own unique game mode. There was a Riot developer explaining that Butcher's Bridge was actually another game mode and not just a map change for Howling Abyss.

Announcer voice packs are probably in the same boat. The code doesn't exist to simply change the announcer voice.