Currently, NBC's offerings go for $1.99 on iTunes (TV shows), and they wanted to double their gain on iTunes by asking Apple to charge users $4.99 per episode. Talk about being greedy, sheesh. Apple, as a result declined this, and has stated that other media companies, such as FOX, The CW, ABC and CBS are quite fine with $1.99 and are on the catalog still.
Eddy Cue, who is the VP of iTunes, has stated this in reaction:
"We are disappointed to see NBC leave iTunes because we would not agree to their dramatic price increase, We hope they will change their minds and offer their TV shows to the tens of millions of iTunes customers."
Nevertless, the NBC aired shows will still be on iTunes up until the end of this year.
Full press release @ Apple