I got a hunch that the team behind prepping the game for new hardware just didn't have enough time to polish the port for DLC as well.
The outrageous prices are provably set by sales and/ or upper management anyway.
I kind of don't get the urge for buying BotW on switch 2 if you alredy have it on original. Why not save your money on new games to play that are coming for thw system?