Fair enough. I figure if you want a steam copy, you buy it there, and if you want an itch copy, you get it there. If it's for the same PC operating system, I'm not sure why you would need 2 game keys for the same game.
From following other developers and hearing their experiences, my understanding is that it costs money to host your game on multiple platforms, and for steam specifically, giving away free copies can hurt the developer because steam doesn't count reviews from users with free copies towards the total reviews which hurts the game's visibility.