Rational choice has been a huge imperialistic success, growing in popularity and being applied to more and more fields. Why is this? It's not because the rational choice model of decision-making is particularly realistic. Rather, it's because rational choice is a completely specified theory of human behavior
, and therefore is great at generating hypotheses. Given any situation involving people, rational choice can be used to generate a hypothesis about what to expect. That is, you just ask, "What would a person do to maximize their own benefit?"
Similar things have been said about evolutionary psychology: you can always predict behavior by asking "what would hunter-gatherers do?" Now, certainly both rational choice and evolutionary psychology don't always generate correct
hypotheses, but they're incredibly useful because they at least give you a starting point.
Witness the theory of bounded rationality: just like rational choice, except amended to consider computational limits of humans and organizations. This is just as imperialistic as rational choice, because it's a fully specified theory of human behavior.
Rational choice and bounded rationality are both cognitive and behavioral models. Rational choice posits a particularly odd cognitive model, but it's still cognitive, or at least lays out a well-defined set of requirements on a rational agent's cognition. The next obvious step is to make more sophisticated and realistic cognitive models that take into account heuristics people use and display biases we observe. Cognitive modelling for social science is the logical extension of the rational choice research program.