The digital version is being removed as well, though. Of course you can download at any point until the cut-off date then that's gone too for reasons only known to Nintendo.
There's some underlying purpose to what they're doing that hasn't revealed itself and won't until after March 31st 2021. In the interim they're causing people to scramble precisely because we don't know what happens after that date. Common sentiment is selling them separately ( since the titles will no doubt exceed $60 if buying all three). I don't recall them deliberately limiting the availability window of the Wii all-stars. More like conservative shipping numbers that probably exceeded expectations and led to a 2nd print run.
I'm curious to know what happens with Super Mario 35 since I would expect that to take-off like Tetris 99 did. Maybe they're planning on selling that next fiscal year as well.