I am not the best person to answer this but I understand that it is mainly because a rotating tyre acts like a pump and is able to remove water between tyre and runway to a certain depth/speed. If the tyre isn't turning it isn't "pumping" and will therefore aquaplane at a different speed.
Thats a very simplistic answer and I am sure someone with more tech knowledge than I will be able to fill in the gaps for you.
