I noticed that pull up resistors are much more common than pull downs, why?
For example, the Arduino's MCU has internal pull ups but these tend to invert the physical logic of things you are working with (such as working with switches) whereas a pull down resistor would do the same job and avoid the logic issue.
Best Answer
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.