No explanation was given as to why they didn’t forgo the key card altogether and just release to the eshop only.
TBH compared to digital-only, having a GKC is still a preferrable option, as it can be sold when you're done with the game.
I think a better option for cases like this would be a cartridge that contains the compressed game data which can be installed to the local storage, as is the case with the other consoles' disks. But presumably the Switch 2's OS doesn't even have that capability right now.