Yoshi's Island is an interesting case of a game that, for a time, was almost absolutely intended to be mainline Super Mario.
The prototype name is Super Mario Bros. 5, internationally the game is Super Mario World 2, and even in Japan the game was given the Super Mario name - Yoshi's Island was the subtitle. You can absolutely see the shared DNA between it and future Super Mario games, too - the use of Red Coins as a collectible (and a greater focus on exploration in general), the idea of Bowser growing to huge sizes for certain boss fights, and the extended Super Star theme all come from Yoshi's Island. And Yoshi's Island was included with the Super Mario Advance line of remakes, which would seem to cement it as a Super Mario game. But the way Nintendo views the game has changed; even with the "Super Mario" name, it's considered the first Yoshi platformer now and the start of its own series.
In a world where Mario Lands 1 and 2 weren't considered mainline for a time (the
Super Mario History 1985-2010 booklet that came with All-Stars Wii doesn't mention them in any region), I'm not surprised that Wario Land wasn't ever brought up in the conversation of being a Mario game. By the time the first two Land games were included with the SMB 30th anniversary material, the idea of Yoshi's Island as mainline was pretty distant, and you can see Wario Land with the same logic as it - namely, even with the Super Mario Land 3 title, it's the first "Wario Land" game instead.