String theory and intelligent design are both attempts to describe aspects of the universe. Neither is feasibly falsifiable (yet). Why does the former attract so many more scientific minds (and more funding)? It's about explanatory power. Scientists may pursue idealistic ends like knowledge for its own sake, but they're very practical about their means. String theory at least promises to lead to an elegant, unified explanation of fundamental physics. Intelligent design, at least as I understand it, offers "the designer did it," which is a dead end when unaccompanied by a program for describing how the designer did it.

John Timmer has conducted an interesting little survey of scientists' use of falsification.