Because they commit to it. Is it any different than us keying in on a passing QB like Gabbert, Foles, Jones, Locker, Cousins, etc.? But to do that, you give (and get) opportunities in other areas. We won 9 games this year and should have won today's bowl, NW, and Michigan also, except for dumb mistakes. Wisconsin most effectively shut down Martinez, forcing him to throw and then disguising coverages (helped out by Beck's poor playcalling). :2cents:
Also, you don't have to say either "he's no good" or "we need to stop Martinez." When asked how to defend Nebraska, they all say they need to stop Martinez. When asked how we would defend SC, for example, we didn't single out their QB.
It must be relatively easy to stop Martinez, because there are a number of QBs that defenses focus on that still have a lot of success.