I find it perfectly fine for game creators to use a different toolset and a different platform to make their game the best it can be.
I don't get why so many people are upset about this, but 1) It's not such a significant price increase, and 2) They should instead be happy that they supported this game early on and caused the game to become what it is today/what will become in the future.
Personally, I will be re-buying the game on Steam, both to support the creator and because I like the game. People should be thankful that games that cost less than 3 dollars still exist, when we live in an era where most games go for 60+ dollars.