Essentially steam has created an almost walled garden. You already have all of your games on the steam client so if a game is sold on another store its inconvenient. Another part is steam has a lot of eyes looking at their store. Everyone checks steam for new games so if you are not on that store then people wont see you.
Its an unfortunate system because steam isn't actually providing that much value but they have just managed to centralize everything on their platform.
Its an unfortunate system because steam isn't actually providing that much value but they have just managed to centralize everything on their platform.