But DJI have never said what their reason for changing the motors was.
Since the originals work perfectly well, it could well be cost or changing suppliers.
No-one knows and everyone is only guessing.
There's some confusion here but I'm not sure it's come from DJI.
The P2 series had the ESCs separate from the main board.
But the P3 series has always had the ESCs integrated into the main board.
That has never changed.
For anyone wondering about new/original motors, there's nothing wrong with the old motors and the new ones don't magically make your Phantom do anything significantly faster, longer or better.
They are just different and both are just fine.