They just want to keep the community as active as possible. A new online multiplayer-focused IP on the poor-selling Wii U. Would you be confident that the community could hold together with that scenario?
Ok, say they release everything. Cool, right? Tons of weapons, 5 modes, about 10 maps and private lobbies. It's the first week it came out. Everyone's having fun. There's 3 maps in rotation because why not? It's easy to find people in the mode you prefer. Awesome first week, right? Let's fast forward to 3 weeks. 1/6 of the online population is gone because they got bored or there's new games to be played. Not much of a big deal, right? It's still fairly easy to find people. Fast forward to 5 months, about half of the initial online population is gone because of whatever reason. People are having some issues getting a game going. Some modes are barren. Mobile sucks trying to explain this. Rooms are getting disbanded because the 8th player didn't show up. People will give up on the game when those issues happen to often. It then snowballs and now barely anyone plays. We also can't forget that Splatoon has competition from Smash Bros, Mario Kart, the Xbox/PS4/PC (for multi-console owners), and even the 3DS itself.
While there's some obviously flawed reasoning in my scenario, I think that the solution they came up with is working well enough to keep the online community steady. Yeah, I have a few issues with the game, but they're pretty minor for me. Anyways, mobile sucks, so I'm done here for now.