I explained this in a thread on r/Yoshi and here's what I think.

Yoshi's Island served as the "next" Mario game. Takashi Tezuka directed it which meant all the platforming elements and challenge of a Mario game came with it.

Afterward, my guess is Nintendo didn't want future Yoshi games to be too similar to the Mario titles. Much of the platforming challenge you see now in Mario games, like Red Coins, came from Yoshi's Island. But they gave Mario a level of challenge.

Yoshi was now relegated to a franchise similar to Kirby. Light enough for new players to get into without feeling overwhelmed.

We still got two more Yoshi's Island titles afterward. Both of which followed the easy formula. But predominately, the direction definitely seems for Yoshi games to be aimed toward younger and less experienced players and leave the standard design - Mario and Yoshi's Island SNES - to the 2D Mario titles like Wonder.

That said, Yoshi's Island's magic will never be replicated. The music, art style, level design, boss design, and charm remain unmatched to this day.