Does anybody know why No assumed temperature takeoff is allowed for the contaminated runway?
A clean runway is just that; a known quantity.
A contaminated runway may have varying degrees of contamination in its length and width. As a result its affect upon acceleration and, in the event of a rejection, the braking action cannot be known with any certainty.
That is why we use full power (or derate) and a flap/slat that will get the aircraft airborne without delay.