Because it's great for a fast straight start, but not for side to side

Frankly, looking back, it was not helpful to start a WR in a 3 point stance.
It is good if a) there is a need to keep pad level low for leverage, or b) if you want to go straight ahead in the fastest start.
But, if you are being press covered, or closely covered, you can't move laterally as quickly. And what if it's a reverse? Or a screen? It slows down the process.

A 3 pt. stance for a WR is a disadvantage to the Receiver......at least in the modern game.