What factors are responsible for the general direction in which low pressure centers move across the contiguous United States?
On the globe there are circulation cells that are set in motion by solar heat. The main cell is call the Hadley Cell (after George Hadley who first explained trade winds). It is set up by rising air around the equator that moves northward (in the northern hemisphere). As it rises it cools and it precipitates as the moist air condenses.
Eventually, around 30 degrees north latitude, it is sufficiently air enough and heavy enough (dry air is heavier than wet air) that it sinks. Since the air is sinking and is hot it is the main reason why there are so many deserts around the Earth at about 30 degrees latitude.
Moving further north a secondary cell call the Ferrel Cell (named after William Ferrel) is established. This cell is actually established by the Hadley Cell and the Polar Cell (guess there was no one to name this one after) and not so much as a cell established by itself. The Polar cell is also driven by heat but in this case the lack of it. Cold dry air sinks around the North Pole and the relatively warm air and relatively moist air at 60 degrees latitude rises.
It ends up looking like this
The last thing to consider is the rotation of the Earth. This causes something known as Coriolis. Essentially it is an apparent force that causes moving things on a rotating surface to veer direction (think of trying to draw a straight line on a spinning basketball). This gives us Buys Ballot's Law which states that with your left hand at the low pressure the wind will be at your back. With the high pressure at 30 degrees latitude and the low pressure at 60 degrees latitude, the general flow between those latitudes will be all from the west.