I don't think it's bending skill which has improved over the last tons of years. It's availability of training an info. It seemed like only the royal family (and very high ranking military) of firebenders were using lightening, which makes it seem like the how of it was a guarded secret. So anyone who wanted to learn outside those groups had to discover how on their own which takes way longer than having someone show you how. Same with Toph and metalbending. Once she discovered how she could teach others so naturally there were more of them.
Basically the potential in people to do these things was there always, not many of them could get over the hurdle of teaching themselves though.