I posted this on the splatoon reddit and i'll share it here as well:
I can definitely see this getting a ton of disappointment. My skype group was arguing over this for an hour, lol. We tried to put ourselves in the developers' shoes though, and tried to understand the reasoning behind this. The reason we came up with makes sense, I think:
Splatoon sold around 1.1 mil; really good for a new game, but not great in terms of competitive shooters. At any given time, of that 1.1 mil, let's say.. 10%-30% are online and playing at any given time? (not counting spike days, such as when a new map / weapon / mode is released, or Splatfest. This might be a low estimate, but I think it's fair - not everyone who has the game is constantly playing it) So, let's say 20% of the players are currently playing the game. That's ~22,000 players. Now, all of this is just guesses guesses and more guesses, but stay with me just for the sake of my point. Some players MIGHT be playing single player mode, or local multiplayer, or just shopping / messing around in the plaza. I'll say that's maybe only 5% of the current players. So for this scenario there are ~20k players battling online. Let's assume that half are playing turf war and half are playing ranked, so that means there's about 10k players in the two separate modes right now. In Ranked, the game tries to match you up with people based on your rank, so ideally there would be separate lobbies for C-, C, C+, B-, B, B+, A-, A, and A+, which is 9 different categories. I would divide the 10k players into 9 different groups, BUT the game DOES make exceptions and pairs other ranks together in order to facilitate matchmaking, so instead of dividing 10k by 9, I'll be generous and divide it by 3. So, generally speaking, the 10k players who are playing ranked are split into roughly three 3.3k groups* (though ideally the game wants there to be 9 groups).
*GRANTED, there are probably more C players than B and A players, and more B players than A players, but this way the math will be neater
Summarizing up to this point, of our ~22k players currently playing splatoon:
~2k in single player / local multi / shopping / in plaza reading memes
~10k playing turf war
~10k playing ranked:
~3.3k C- to C+
~3.3k B- to B+
~3.3k A- to A+
If ranked let you choose your mode of play, the groups of players in ranked would be divided by 2, assuming the same amount of people prefer tower control to splat zone:
Splat Zone:
~1.6k C- to C+
~1.6k B- to B+
~1.6k A- to A+
Tower Control:
~1.6k C- to C+
~1.6k B- to B+
~1.6k A- to A+
And then with Rainmaker's addition it would be more like ~1,000 in each. Even less than that for A-ranked players.
"1,000 players playing the same mode at any given time? That sounds like it'd STILL be easy to find a match." It does SOUND like a lot, but consider: of those 1,000 people playing, many of them are likely /already in a 5 minute match/ or otherwise have a complete lobby. Unless a lobby frees up, you might have to wait ~5 minutes for a match, possibly longer. (Or, hopefully this wouldn't have to happen, but you might join a lobby before someone who was /already in it last match/ could join it again, and now THEY have to wait, and then someone takes YOUR spot the next match, etc).
I might have done some bad math, or made really bad assumptions, who knows, but I think this was the developers' intent with this design choice - to not split the playerbase and keep matchmaking times short. I mean, I do wish you could just choose. I think the developers wanted to do that too, honestly, but when they thought of it this way they took the more conservative route.
Edit: Looking over the reddit thread for this I think
Alecat was thinking along the same lines but said it better!