Howling Abyss and the Butcher's Bridge are just Map Retextures, though - they're not new Maps.
A New Map would mean a new Terrain Layout, and, to make it anything exciting, require significant features in its Cast of Items, Jungle Set-Up, and Map-Gimmicks (like Baron, Altars, or Capture-Points) - enough to make the Game-Mode feel fresh - but also solid enough to support constant play by the User-Base over a long period of time - like Summoner's Rift. On top of all this, it would have to be Balanced for all 120+ Champions (which is no trivial task).
And, most importantly - It would require its own Queues.
This is certainly the hardest part, because every time a new Map or Mode hits the Servers, it splits the Community into subdivisions. And, if there are not healthy-enough Player Populations for each of these Queues, then it becomes a downward spiral of increasing unpopularity as Queue-times increase as the Population struggles to stay afloat - and the _ENTIRE _ effort of making the new Map / Game-Mode takes a steady path down the tubes - at least in the relative sense to League's Already Healthy Queues gone right - being Summoner's Rift (which has Several Sub-Queues depending on Mode) & Howling Abyss. From my understanding, Twisted Treeline's Populous is small in comparison.
But then there's Dominion...
Poor Dominion is what could happen to the next Map/Mode - if things were to go wrong.
It has only 2 Queues - one of which is literally dead (Draft Dominion) - in that, if you join, you will NOT be able to find a game, unless 9 other people with unrealistic expectations decide to join at the same time...
And the Regular Dominion Queue is already pretty long, due to the small Player-Base.
THAT is why Riot is reluctant to expend all the effort required to even TRY a real New Map.
But, say even then, with the whole boat-load of risks, the Gamble pays off, and it's a new Hit Mode!
What will happen, then?
It'll suck Players from the Other Queues, making those Other Queues less healthy.
How come?
Well, if it's a really successful Map/Mode with great Depth and Replay-Value just like Summoner's Rift, then it will require not just Normal Queues, but also Ranked (which is needed for a Strong Community), and possibly Draft [which helps foster the Ranked Community by giving them a more laxed environment free(er) of the Casuals in Blind-Pick].
If this happens, these Queues will require a good number of Players, which will siphon yet more Players from the other Queues - and if we're talking about something near or at par with Summoner's Rift, that's going to be a huge chunk of the Player-Base, which will dilapidate the other Queues.
So what if it's just ok - not attracting too many or too little Players.
Well, in that case, from a Business Standpoint, it probably wasn't worth it - and you have to remember, Riot is a business after all. There are other ventures that are less risky and expensive (although, seeing the amount of content they put out, I can't help but imagine the Riot Offices running similar to The Office). So basically, there are 3 Kinds of Outcomes:
- The Map/Mode dies quickly and the Whole Effort largely goes down the tubes - effort which could have been spent else-where.
- The Map/Mode is a huge Success, spawning a healthy Player-Base for it - but at the expense of the other Player-Bases.
- The Map/Mode is kind of popular, but doesn't really make it that far. Good job, you've given some Players a bit of extra fun, but not enough replayability to keep them hooked for ages.
So to recap:
- Wasted Effort - Bad outcome for Riot and arguably the Whole Player-Base [Likely]
- Good Out-Come for Some Players, but not Riot. [Least Likely]
- Mediocre Out-Come for Some Players, but not Riot [Likely]
