No reason was given on why the game is no longer getting a cart release on Switch, but I can only assume it might have to do with a combination of 1) Switch cart shortages as a result of the pandemic causing some companies to be given priority over others, or 2) Panic Button is simply unable to get the game in a state where they can fit most of the data on a cartridge so they have instead opted to go digital-only so there's less data compression involved (similarly their Switch port of Wolfenstein: Youngblood was also digital-only on Switch, but it didn't seem to affect sales that much so it's possible Bethesda figured "if it worked once it can work again").
I do still want to get the Switch port of Doom Eternal as having a portable version of the 2016 game is awesome, but I admit that knowing there's no longer a physical cart kinda sucks.