I'm telling you, they're going for a "let's just have fun and play together" feel. If you win, cool, if not, maybe on the next round, but regardless everyone is having fun.
This is my exact thought.
I understand the want for party chat of some sort, entirely. It can be an enjoyable feature in team-based games. However, it has its bad sides, namely the gigantic amount of stupid/ignorant people (young and old) with a mic. If party chat was in Splatoon, it would have all of those common problems, even if it was limited to chatting with your own party.
And I think that's the problem with the feature, in Nintendo's eyes. They're not focused on having a hyper-competitive game with Splatoon, at least not in regular online matches. They probably want everyone to just "have fun and play together" as Gabe3208 says, and the negatives of voice chat would ruin that for a lot of people.
"What about options?" you're thinking. Those who don't want voice chat aren't forced to have it, and those who do want it get what they want. Sounds great -- best of both worlds. But, if you make party chat opt-in, then it becomes possible for one team to be better coordinated because more of their members have chat on, making it an unfair match. Everyone has mentioned this already, but, what's more important is that
losing unfair matches over and over is just as off-putting as the idiots harassing others over mics.
So let's say I'm someone who keeps party chat turned off. "Okay, cool, don't gotta listen to those idiots now. Didn't want to be told by a fourteen-year-old how bad I suck; I already know. But, shit, this team is smearing us. No chance. What's the point? Oh, this team is just as coordinated as the last. They're clearly communicating together. Man, this isn't any fun anymore..." And so on. I know the teams rotate and whatnot, but that wouldn't 100% fix the problem.
Basically, if party chat was available and opt-in, it would have a very high chance to create a negative experience for those who don't like the feature, whether or not they opt-in. And that's what Nintendo wants to avoid. They want their game to be competitive, yes, and it will be to a certain degree because of the game's balance and the level playing field without party chat. But, more than that, they want the game to be fun for all different types of players, and to do that they need to avoid the negatives of party chat.
I'm sure they know exactly what they're doing. They're sacrificing a slightly higher level of competitiveness in order to make it fun for everyone. "Lets just have fun and play together." I'm sure they know they're also sacrificing potential sales from people who won't play it because of the lack of options. But, this is the type of game they want to make. It's clear from everything they've said and shown about it. It does suck if you really enjoy party chat in all your games, but you have plenty of games you can go chat in already.
Get over it and come enjoy the uniqueness of Splatoon. Or not. Doesn't matter to me.
To me, it's just a different flavor of game. PS4, Xbone, and PC all have flavor A covered. Nintendo wants flavor B. I happen to be a man of many tastes, so I'll gladly give Nintendo my money for a different flavor.