It's the same as the problem with Saturday Night Live.
The sketches aren't funny.
Aaron Sorkin, we learn, has a weakness -- other than crack cocaine, of course -- and it's sketch comedy. What makes the Sorkin thing work -- fast-paced, pedantic, clipped dialogue -- for whatever reason just doesn't translate in the sketches on S60. I can't quite pinpoint the problem, but I have a feeling Sorkin relies too heavily on an audience's relationship with and empathy towards his characters, and you don't get to build that in a sketch. Or maybe Sorkin himself relies too heavily on his own relationship with and empathy towards his characters, and he can't write solid one-liners for two-dimensional characters he'll never see again.
I gave him a pass last week on that Pirates of Penzance unfunniness, thinking he just had to get his feet under him, but it happened this week, again. I really, really like everything else about the show, but if the sketch comedy doesn't actually become funny at some point, I'm going to have a lot of trouble tuning in each week to suspend my disbelief. Maybe time to change up your own writing staff, Aaron?
Update: America agrees with me. S60's ratings are down 30% from its premiere, 17% from last week.