Ladybird was originally started as a browser for SerenityOS, a POSIX operating system. Well into the project, they decided to make it cross-platform but that still meant POSIX ( Linux and macOS ). As interest ( and sponsorships ) came in from outside SerenityOS, focus moved more and more to the browser and away from SerenityOS.
Just recently, Ladybird decided to split from SerenityOS, allow more outside code, and in fact has dropped SerenityOS as a supported OS.
The project is fairly pragmatic. I am sure they will add Windows support as the core browser engine matures.