NMOS - Why is a resistor necessary?
N channel mosfets when used in a typical switching configuration connect their output to ground when turned on and leave their output disconnected when turned off.

So with a pull down the output will be permanently stuck low. With no pull at all the output will switch between ground and floating. In the absense of any external influences a floating output will generally stay where it was last set so again the output will be stuck at ground.

Note that there is a trick involving a N channel mosfet with special geometry that can be (and usually was) used as a pullup instead of a resistor.