Ryan Sager's guess:

[M]y theory of why no one in politics likes to think about political science: because it renders them powerless. How do you do your job as a political consultant when the truth is that 90% of the success or failure of what you do will be determined by the unemployment rate? If you’re a political journalist, how do you write a story every day for a year (or three years, given our current presidential election system) saying, essentially, “Well, the fundamentals still make it exceedingly likely the president will be reelected.” If you’re a politician… well, then you’re a sociopath anyway, so perhaps it’s not worth getting into this scenario too deeply.

And then there's the fact that so many political scientists are quant-wonks you'd run from if you met them in a Starbucks. And, yes, I have a PhD in political "science".