It's worth noting that condition variables are a finicky interface. Speaking personally, I would typically much rather work with something like a semaphore. I also feel like I've seen a lot of misuse of condition variables out there.
Semaphores alone can’t mimic the same semantics (juggling between multiple, disparate locks/sync objects) without running into a possible race condition unless you have an API that can somehow wait on one of two or more semaphores to become available.
A Win32 mutex or semaphore is very heavy and typically avoided in a sane high performance application unless you need it to cross process boundaries. A lot of the time (most?) you will want a win32 critical section, which is similar to the modern futex based pthread mutex implementation in that it does not enter the kernel unless there is contention.
A pthread condition variable is clumsy. You need to give it an extra mutex and loop etc. It doesn't have any of the simplicity of WaitForMultipleObjects, even with all the faults of that interface of which there are many.
My understanding is that condition variables were designed to be a superior alternative to Windows Event objects.
I'm glad to read here that others find them fiddly and overly awkward to use since that was also my impression vs the consistent and usually easy to use Windows thread synchronization HANDLE based objects. Dave Cutler definitely did a good job there IMHO.
In particular pthreads borrowed them from Mesa, but they originally were part of the monitor abstraction designed by Hoare.
So they weren't designed to be a superior alternative to windows events; they might be considered an alternative to events in general but I couldn't easily find any history of the events abstraction.