The more I think about it, the more I realize that 3D All Stars is such a bare minimum slapped together thing. The only scenario that makes sense to me in my brain is that it was meant to be a celebration/promotional limited release, like the 25th Anniv collection on Wii, and were the collection revealed during E3 or on Mario Day (whichever) in Alt-2020, it would have been paired with other massive announcements that would make it seem more like a digestif than the main course.
Fatmanonice
has implied as much, and I'm inclined to believe it at this point. There's no way this was meant to be a full retail swan song for 2020 kind of game, and it's really looking that way. This wasn't meant to be the "showstopper" is what I'm saying.
And I would not at all be surprised if the 3 games appear separately on the eShop after the limited release period is up. It's definitely something they would pull, maybe even reveal them appearing on there as digital only like... 3-6 mos after the limited run ends, as a sort of "Hey, look what we're giving you. Sorry you missed out, but take what you can get!"
EDIT: 3D World + is definitely not the "showstopper" either, but it feels like it got all the attention. According to Emily Rogers that additional content is extensive and arguments about compression aside the game is almost 3x the size of the Wii U release.