WebReplay requires recording and replaying a lot of low level system specific stuff, like system calls, IPC traffic, stack, and the heap. The "Porting" section of the linked article talks about this. I think the design is intended to support working on different operating systems (as compared to rr which is likely to only ever work on Linux), but porting will require additional work.

Probably because the work hasn't been done yet. It's a very small team.

Yep. It's on the roadmap, but the recording is done at the OS level, so Windows and Linux will require some work to get support.