The reason is that you start producing lift at rotation and you stop producing lift at touchdown. I mean that the wingtip vortices are a direct consequence of the pressure difference between the upper side and the lower side of the wing, as you know, and the airflow passing over the tips of the wings trying to equalize those pressures.
You start producing significant lift - wingtip vortices- at the rotation point. And you stop producing significant lift - wingtip vortices - when you touchdown.
And the greatest vortices (wake turbulence) is when the a/c is :
slow
clean
heavy