Most likely because Windows can be adjusted/modified more easily and has the most knowledge of the developers....A question for the Taiko team regarding the software choices and architecture:
Why Windows and not another OS?
I'm thinking for instance of a BSD based system which license would allow the development of closed source software ( guaranteeing the commercial aspect and the intellectual property).
Or a Linux, more widespread, but whose license imposes constraints.
This would allow to have a solid and perennial software foundation (OS and userland).