In FPS games the map is just a stage where any objective (capture the flag, control points, etc.) can be copy/pasted in. They usually have many game modes with no physical objective, including Deathmatch, Team Deathmatch, Zombies, etc. As long as the players have somewhere to run-and-gun that's somewhat interesting and somewhat balanced, the map is doing it's job.
Try to imagine if League worked that way. You've got no Turrets, no Inhibitors, no Nexus's, no minions, no jungle monsters, no Baron, no Dragon, no Vile Maw, no Alters. Such a game mode would be able to have almost limitless ways to redesign the paths through what we now call the jungle, because there's no objectives to center the conflict over.
Such a mode also wouldn't feel like League of Legends at all.
League is first and foremost a game about objectives, primarily Towers, Inhibitors, and Nexus's. We've had a grand total of one permanent game mode that didn't follow that format, and that was Dominion, the game mode that was played by so few people it was demoted to a Featured Game Mode. If it's going to be a map for League of Legends, it needs Towers, Inhibitors, and Nexus.
Towers need lanes, so any map would also need lanes. Lanes need to have something separating them, which has lead to Jungles. All of the current permanent maps have a set number of lanes and jungles:
- Howling Abyss: 1 Lane, 0 Jungle
- Twisted Treeline: 2 Lanes, 1 Jungle in between the lanes
- Summoners Rift: 3 Lanes, 2 Jungles in between the lanes
If a map copies the number of lanes and # of jungles from another map, it'll likely be considered a re-skin, providing an extremely similar experience. Thus if you want more maps for League, you've got to add more lanes/jungles.
"Fine," you say, "Lets just add more lanes!"
Well, how far can you carry that? At 4 lanes you've gotten rid of the support position, making 4 solo laners and 1 jungler. At 5 lanes you've gotten rid of the jungler, having 5 solo laners instead. You've hit the physical maximum number of maps without dramatically changing the objectives (which again, our friend Dominion didn't have lasting success with).
"But what about Heros of the Storm, they have tons of maps!"
Heros of the Storm maps are focused around the neutral objectives. Teams spend a lot of time fighting for the neutrals. When was the last time you decided what map you wanted to play based on if you wanted Alters+Vile Maw in your game vs Dragons+Baron Nashor? Likely never, and it's because the focus of League Maps isn't on the neutral objectives.
TL;DR: League's pretty much capped out on maps. If Riot's really cleaver there might be a way to add one map, but certainly not 10 more.