RBY Gen I —> FRLG (idk) missing Yellow
GSC Gen II(?) —> HGSS missing Crystal
RSE Gen III —> ORAS Gen VI missing Emerald, added XY
Gen IV's DPPt had no secondary series, and B2W2 was an "upgraded" version of BW, just like SM —> USUM.
There is no confirmed answer to this, but my guess is that some of the newer games didn't have enough content that could make a third game special enough to justify making one, when you could save time by making two. Most of the games that came out after they took away a third game excluding X and Y have gotten remakes as the games after them like Black and White 2 Ultra Sun and Moon, so I don't think the concept is completely lost. Hope I helped cure some of your curiosity I'm sure there'll be other answers with other points of view.