I think one of the major barriers against increasing the number of maps is the lack of interest there will be in them. We already have 3 game modes (4 if you include ARAM matches). What is the most popular? Summoner's Rift by far. What do ALL OVER MOBA GAMES copy? A map style similar to 5v5 style of summoner's rift.
Looking at it from a competitive sport's stance, I think Riot is making the right call, at least on the decision to stand by a singular map and competitive arena. Basketball, Baseball, Football, etc. while there are thousands of fields, the same basic qualifications for how their fields are structured remain in place. Don't try to reinvent the wheel. If you're bored with League, it is probably due to a lack of ability (in general, or with a wide variety of champions), Go play another game, it's not like you're wasting money on riot, it's a free to play game.