Because it is a different type of game. With online centered games, your experience (typically) involves doing the same thing repeatedly, with the other people making the experience different. With single player centered games, your experience (typically) involves doing a lot of different things and not really repeating yourself.

A spread out launch schedule in an online games keeps all the players focused on one thing, and as excitement for that one thing begins to diminish, new elements are added, which revitalize the community (in Splatoon specifically, it seems that they will release new maps fairly frequently as well as new items basically daily to keep players coming back, as well as the more major mode updates). With a single player game, if you run to the end of the content given, you simply stop playing the game. This presents numerous problems with pacing, maintaining interest, keeping the skillset maintained etc....

Episodic games (in response to your response to me) tend to be a little different as that implies a single player game, however, they typically are designed to give each piece its own feeling to give you some closure at the end of each "episode". This is like how many TV shows do things, which keeps the player interested in what comes next, without really breaking the pacing. A single player game has to be designed around this idea to work, and it typically works best using story to hold the episodes together (instead of gameplay) so it can't really be shoehorned in.

With typical multiplayer games on the other hand, because the experience is inherently different, it is more suited to a spread out launch schedule (for the reasons stated before). In multiplayer games with a boatload of content upon release, it isn't unusual for a player to find the piece they like the most and pretty much exclusively play that piece, thus spreading the community and having players miss out on other content. This also weakens longevity, as when they get bored of that mode, they consider that to mean getting bored of the game. Games like CoD can sustain this because they have a giant playerbase and a ton of legs, but smaller games often struggle with keeping community interest, so Splatoon's release schedule (if they map out their releases well), could be a boon for all players.

On the other hand, releasing a game lacking on content (1/3) could also spread firstly a bad impression and people skiping it. Because of that, your current userbase could be as low as to make the the first players get early bored. Thus, they will lost any future interest in the game and won't bother with the subsecuents modes months after.

I guess we will see how will people react to it.

It could and that is a very real risk. Evolve went the route of releasing low on content and trying to keep people going with post release content (although they charged for their post release content) and its user base dropped extremely quickly. It all comes down to how you release content and if you release it at a pace that lines up with when people begin getting tired of the current content.

That said, a lot of people are making this strategy out to be evil or terrible, but while there is potential risk, there is also potential reward and it could end up making the experience better for day one buyers. We will see how it plays out, but I think that judgement should be made a few months after release instead of immediately just saying that it is stupid and the game should have been delayed

Sounds like a really cool group of guys!
And I really enjoyed what I played from the Testfire, and what I saw at E3 last year, so I'm definitely getting this game, day one.

I don't think any option really makes it definitively better for day one adopters. Personally, I see a strong middle ground happening with Splatoon: Some people will burn out because one new map a week is actually TOO much, others will become addicts, etc. I personally would've preferred them to just throw a couple more maps into the base game and spread out the new maps a bit more. One every other week or every three weeks, with like five more maps at launch, would've been ideal. The only reason I can see for them not doing that is to tweak the dlc maps to the playerbase's tendencies a bit (remove clear camping opportunities, inevitable glitch spots, advantages, etc.) to ensure that later maps are better than the launch ones. If that's the case I'm fine with it. 

It's gonna remain to be seen how the reviews go with Splatoon and if reviewers continue to update the reviews as the game evolves. Since this probably isn't a game that'll have a broken launch (MCC), and it's not a huge blockbuster (Destiny), I don't see that happening. So the Nintendo fans are going to have to push this game for Nintendo this time, as the reviews probably won't help out this time. 

Iwata Well, I guess ink-an’t be helped. (laughs)

*green sky laughs*