"What happened to the "third game"?"

Ever since Platinum in Gen IV, we haven't had any "third games". For instance, there is not Pokemon Gray, Pokemon Z, or Pokemon Stars, although the presence of Black/White 2 and Ultra Sun/Moon in Gens V and VII are a kind of followup to the first pair of their generations.​Has there ever been given a reason for why there haven't been any "third games" in the last three generations? Do y'all think they'll ever come back? I've always liked how Emerald/Crystal/Platinum patched up issues, had more pokemon variety, and spruced up the content. via /r/pokemon https://ift.tt/2Qi6da0