One reason is that NBC is flailing, grabbing for straws, and has been now for years. ABC was in that position, and Lloyd Braun brought in Lost and Desperate Housewives, which brought them back up above water. The only reason we are still seeing eps of 1600 Penn is because the cupboards are bare; they have no better product to put on if they cancel it. They are obviously in flop-sweat mode; they have pulled eps and reinstated eps for that show at least twice since the sweeps period began.
... it begs the question, how did this stinker get made in the first place?
NBC has a long history of throwing spaghetti at the wall and hoping something sticks. ... They need to repair their reputation and put some better shows on the air.
DNH was a case of the cupboard being otherwise bare, which leads to the spaghetti scenario. This is also why a moderately successful show (cough! The Good Wife cough!) on a successful network (cough! CBS cough!) is in danger, which is that they have lots of young strong shows in the wings itching to be placed in front of an audience. That, or CSI:NY, or some other beloved show will have to go in May, maybe all of them. NBC would kill to have a show like TGW even with its marginal ratings.
To get some contrast on just how far NBC has fallen, read Top of the Rock by former programming chief during Must See TV days, Warren Littlefield. The Conan/Leno fiasco is just the tip of the iceberg that brought them down. Most think Jeff Zucker single-handedly scuttled NBC. Being owned by a cable operator doesn't help, either.