Why are pull up resistors more common than pull down resistors?
TTL has a threshold between low and high that is closer to ground than to the positive rail, so it is better when the stronger transistor pulls the output down against the relatively weaker resistor.
In general ground is presumably a better (e.g. more stable) reference voltage than a power rail.
You can use open collector/drain outputs as voltage converters, if you connect the resistor to the positive rail of the target voltage.
The ancient resistor transistor logic even used this as its working principle throughout.


That said, some microcontrollers have configurable internal pull-ups and pull-downs, e.g. the NXP LPC1xxx.