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My argument here is that the NES was starting point for Mario and at the time, Nintendo was experimenting with their IPs as they weren't as massive as it is now. You are however correct that they did do more than one main game in the NES, SNES and GB era.
However, the SNES only had one new mainline game. All-Stars is rather a port of the classic NES games. As for GB having more than one game, I can only make a theory that Super Mario Land was extremely short of a game and so they had to make a sequel to improve the portability of Super Mario on Game Boy. Wario Land is more of a spin-off that had nothing to do with Mario other than a cameo and the title for a good marketing scheme.
Yoshi's Island had no reason to mention Super Mario World 2 there because..its a different game on its own and it was just there for the marketing reason.
Once Mario started on the N64 side, there has been only one mainline game per gen and it has been like that since.