[ComputerGuru]

It is because condition variables have no state, i.e. calling signal without waiters is equivalent to doing nothing and does not affect a future wait. The concern is that a thread is switched out after it checks the condition but before it calls wait, and in that moment the condition variable is signaled. The thread then calls wait and is not awoken (potentially never to be awoken). If the predicate is not altered without the mutex and the waiting thread has the mutex at the time it checks the predicate (this latter part is a must in all cases), then it is guaranteed the predicate will remain unmet at least until the thread has switched to a waiting state.

So simply using an atomic (even with a full release-acquire memory barrier) is not necessarily sufficient as that would only enforce the coherence but not prevent the scheduler from interrupting at the wrong moment.