Because they're developing Steam version of Raft that they're going to charge 19.99$ for on early access release coming in 23th of May. The fact that they've stopped the development of itch.io version has been written on the game page for over a year now you know:
"Raft will be released on Steam Early Access on May 23, 2018, with multiplayer, reefs and much more new content and features! To be notified when it launches, please visit our Steam page and press 'Add to your wishlist'! We are not releasing any new updates on Itch.io as we are working towards a Steam Early Access release. To read more about this announcement, visit our Blog!"