I would also add in looking in terms of the show and the time travel aspects
If you look at America historically (and to the present day) the chances are fairly large (and they grow significantly the further back in time you look) that the person or people in charge are white men.
So if you are being realistic to storylines and plots and you have some type of figure with authority who is the 'villain/antagonist' in the plot the chance of that individual being a white male should be fairly high.
It would actually be unrealistic to have someone leaping back in time and put in situations where "oh, leaped into a doctor... That evil money grubbing hospital CEO is actually a black guy"
"Oh leaped into a coal miner... That evil money grubbing CEO is actually an Asian guy"
Unless he leaped out of America.