Many people prefer to buy via itch so they can be sure the developer is getting a the percentage they chose, rather than one imposed on them by a platform-holder. This is one of the major attractions of itch for a lot of people, including myself.

I just bought it here because I've been excited about it forever and because sale. However, it would be lovely to have a Steam key because I'm slightly more confident that Steam will keep track of everything I own for me.

That would be really nice although I've bought the game on steam already :-P But other people would probably be more eager to buy on itch if the key is an option.