Because it had all of the right features.
It was based on a compact car (Falcon) so cheap to make and sell.
It had four seats so it was practical.
It could be economical (200 I6) or sporty (289 V8).
It could be a coupe or convertible (and later, fastback).
It had loud styling which appealed to a younger generation.
It was marketed and released just as the first Baby Boomers were coming of age.
It was priced that it could be a second car, a kids car, a first car, or a car for someone downsizing.
Here I am surprised that it was still alive.