I want to buy visual novels, and I want to buy them on itch. This is hard when a sale happens and vns are on sale for double the amount of steam - think 60% on steam and 30% on itch. I end up gravitating toward vns that have roughly the same discount between websites, but that tends to mean always buying the vns of the same dev or two. 

Why do devs do this? If they choose to discount a game by some amount on steam, why are they unwilling to do the same on itch?