I kind of put it down to combination of speed-of-plot, *and* Aang having already mastered one style of bending so thoroughly. If every style is a matter of control of chi through breathing and movement, once you've managed to figure out how to move one element with them, it could- theoretically- be easier to learn how to apply it differently to another element with proper instruction. I get the feeling they were really looking for broad strokes that could be made up for with raw power, and if they all survived, Aang could build on the basics later.

But yeah, sometimes I like to stretch out the timeline to a year and a half or so^^

Jet: "So... the Fire Nation is now trying to be good, the Firelord is a chick who actually helped stop the war, and my *kid* with her is going to be heir to all this insanity.

"Guess I'd better get to hunting down those dissident groups that aren't falling in with the non-conquering program, hadn't I?"