Because people could have ordered it from another site or, bought a different version. You can't even buy any version of Zelda on the site anymore. If you bought a Switch and can't find a copy of the game, your only option is to buy it digitally which will basically require an immediate purchase of a MicroSD card.
If there was some kind of clause of 'we may not be able to fullfill this order but we will try our best', I would have made a backup order for the normal SE when it was available. Now I have to settle for a normal version from BB, a competitor of theirs.
I have never, ever had this happen through BB or Amazon, it's not exactly something we should assume. It sours me from ever relying on them again that's for sure.