Because you need a certain amount of notice and in this case drama to be created so people know to update ASAP, before the exploit is out in the wild. As soon as these patches are out it becomes a lot easier for any actors to figure out exactly what they are patching, and how to exploit it.

Announcing when this patch is going to be released should leave a much shorter vulnerable window with the exploit out in the wild than releasing it immediately and hoping people update before they're a target.