This: "Flash had a very unique set of functional/creative affordances that have never since been replicated in an authoring tool".
It was extremely to do a UI application with Flexbuilder, very similar to using VisualBasic 6.0. Just drag some widget and connect the event to some functions.
Also drawing and animating vector graphic with the Flash authoring tool was extremely easy.
Today there is still no software that is this easy to use.
You dont see 7 years old kid coding vector gfx animation with javascript and css :)
But back in the day it was not uncommon to see complex flash animation designed by 7 years old kid.