Coming from the indie dev scene a while back, as an indie dev, you are typically burning every single hour in your day making a game, begging for money to get the game to market, pitching your game to publishers, or screaming into the void that is social media in hopes anyone will click your link. You simply do not have time or effort to spare. It's a hugely saturated market and the currency is public attention.

So, you tend to cast the widest net possible in hopes you get some kind of traction, which means marketing to Windows users. I'm sure plenty of devs would vastly prefer to be on and build for Linux, but the fact of the matter is the marketshare is smaller and Proton exists.