It's not so much nostalgia as simple name recognition. Lots of people have heard of Spelljammer, maybe seen a thing or two about it somewhere or other, and so, they see a new Spelljammer book and they already know, at least in very broad terms, what it is.
If I release something completely new there isn't any of that, and I have to start completely anew building that brand. Lots of people went to see the Pokemon movie who had never played the game nor watched the show. Heck, the MCU is entirely built on that audience - people who have never even seen a Marvel comic book, but, still, through cultural zeitgeist, know who Iron Man or Thor is.
It's not like the MCU isn't popular in places like China or Japan where Marvel comics and Marvel cartoons might as well not exist. But, people still have heard about these names, so they resonate.
The slow release schedule is absolutely meant to maximize profit. 100%. The fact that it has worked FANTASTICALLY well shows that it's not a bad way to go. Ten years in and we've still got less books for 5e than we had for 4e. Book of the Month style production just doesn't work. Took much chaff. And not enough time for people to adopt ideas and for those ideas to ground themselves in the general awareness. Look at
@Minigiant's list of classes for 4e - that was released in what, over a 3 year period? Less? There's just no way anyone would be able to look at, much less play, even a fraction of that. It's totally overwhelming.
So, WotC goes back to the old names - Spelljammer, Dragonlance, Ravenloft, whatever - for that sweet, sweet name recognition. But, instead of shotgunning the market with book after book after book, each one cannibalizing the sales of the other, they make each one a BIG DEAL. Tons of marketing for each and every release. It really is rather impressive.