well,
- Steam takes an unreasonable cut of money for what is functionally a downloadable website most of the time (itch takes practically nothing), so need to compensate for that
- Steam makes paying what you want for games as a easy option to access much harder to do, so theres far less incentive to make it that
- Steam is a horrible platform that promotes children gambling, among other things, so I'd think if possible every developer would want to incentivise moving away from it
- Steam has a publishing fee that needed to be gotten past, itch does not
and I'm sure several other possible reasons
Buddy, steam has been around since 2003, it does not promote child gambling more than any other game does. its not steams fault entirely. sure they made csgo and tf2 and host the games on steam, but you are never forced or even told to gamble with cases or trade ups in either game. also this is something games like cod and battlefield have been doing for a while now so don't just blame steam.
Also steam is the biggest gaming platform in the world, why the fuck would you not want to put your game on it? it makes no sense that you would purposefully gimp your popularity and chances of people finding then buying you game by not putting it on steam. steam takes a 7% cut of your games profit, sure. but just putting your game on steam increases profit by up to double. there is literally no reason to not release your game on steam.