Timing is a factor, too. For such a thing to be successful, you also need a OS/hardware combo that can draw the notification without slowing down using the main window (rules out early Mac OS) and users who think the added distraction of notifications is worth it. I would that added distraction goes down with screen size. I doubt it would have been a success on 640×480 displays, for example.

Also, Apple had something similar in 1997 or 1999, with Mac OS 8 or 9.