Much like combinatorics, it's just not a sexy field of math to study. It's a collection of problems whose solutions don't really build upon each other to build a more general theory. In other words, the solution to one game theory problem typically can't be used to gain understanding of other problems, and so the effort is not rewarding. Because of that, it's treated with contempt by mathematicians who are drawn to more insightful topics like topology or algebra.
