nambo1,
First of all, the real hero is the Flash Player. The Flash
Player, the features it has, and the environments it runs in and
how it runs in those environments (particularly the web browser,
but as kglad pointed, it's very flexible and is used other places
as well) is what makes it so popular. ActionScript is simply how
you programatically use the Flash Player. ActionScript is an
ECMAScript language of which there are others, but it's the Flash
Player (and what you can do with it) that attracts people.
The Flash Player has been popular for awhile now. AS3,
however, is the latest and completely redone language for the Flash
Player. Because it's new and because there are significant
differences between AS3 and its predecessor (AS1 and AS2), there is
some adoption time required. Many people are still using AS2. You
can google to find out a lot more detail about the
advantages/disadvantages to AS3 vs AS2, if that's what you're
really looking for.