Honestly... especially with a new IP.. if you have 33 weapons to choose from I know for sure I wouldn't take the time to test each and every one. I'd look and see what others were using and pretty much play with that. A lot of choices like that is not always a good thing up front cause it can easily be ignored. By getting a new map, weapon etc. here and there, it gives the player base an opportunity to at least give the new content a try and see if it's for them.
It also allows the devs to see how it works and plays out in the game. Obviously they didn't have enough time to play test everything to their liking so this is a way they can better track what may need tweaking of fixing down the line.
Oh, easy. If Nintendo unlocked it all at launch many people will play the game once and put it down and forget about it. But this way players will be encouraged to play the game every week or so to see what's new or changed, and at the same time get more drawn into the game and liking it.
That's why devs implement unlock systems. The more you play the more you unlock. There are a variety of ways that can be implemented and I would say they're much better off doing that than just giving players content content over time without having to earn it.
What about after they've released all the content? People that then choose to buy the game just get all the content from the get go. Had they implemented a proper unlock system that could be avoided.
It just seems lazy to me, if it were planned better they could have handled this a lot better than just periodically giving players more content. Maps I can see how spreading out the release of each could help maintain the playerbase but with weapons and other content I don't agree with the approach.