Part of it is the same reason any live game would disable Linux. There is a LOT of money in premium currencies and RMTs but the game also needs to let players "earn everything" so that folk defend the design. And the studio (shareholders) just don't think they have enough resources to properly test the "proton version" of the game. So if an exploit is found? You can bet the market share of linux users would go up an order of magnitude or two over night as people want to get their exploit cash and break the economy.

And considering that Linux is still single digit percentages of the Steam userbase (and that that is insane considering where we were a decade ago) we just don't matter.

The other side of things is more tinfoil but I wouldn't be shocked if it comes out that one of the other platform holders paid for some form of pseudo-exclusivity. Because the Steam Deck is incredibly powerful in that people are quite likely to want to play the PC version "just in case" they want to play it on their steam deck during all the public transit they do in their every day life. Like, personally speaking, it killed Gamepass for me. Because while I love the idea of being able to just try a bunch of different games to see if I like them, I found myself buying too many games because I wanted to play them on my Deck on my deck (hee hee or to be able to continue my progress when I go on travel.