When I started at Apple it was to work on Quickdraw GX.
It was very cool tech. Closer to what Apple would get from NeXT years later.
My guess is as to why it failed (and this was also the common sentiment at the time) — it was an optional install and not a default. It's hard to imagine anyone writing an app around a component (extension) that few people had in the OS.
The resuscitation was truly miraculous.