Simply put, generations are not based on the TV shows. They're based on the toy lines.
There was the 80s MLP and MLP: Tails, but those all had toys under the same Generation. G1 & 1.5 if you will.
G3 had no show but a series of animated shorts and two movies. Also two, arguably three revamped toy lines. But all with the same characters and setting.
Same thing with G4 and G5. The change between the two was all toy based.
Generation 2 was the first time the toy line changed. For whatever reason, there was no TV show or movie. Just one video game, which granted, was the first ever MLP video game. Demand was low in North America but stronger abroad. So they saved money without making a TV show and just kept making toys until those markets dried up and they took another crack at the states with Gen 3.
TL:DR - It's all about the toys. No TV show or movie, nothing to talk about.