Hi guys. Needless to say I saw this topic and was compelled to join VGC. Let me tell you right here and now, the fault goes to NBC first and foremost.
Now, when Leno was taken off the Tonight Show, he was averaging 5 million viewers a night. He was #1 for years and probably would have still been. Now, it is fair to expect Conan to at least maintain the viewing audience. The fact of the matter is at the same time slot, he lost 2 million viewers. If he had been able to hold the audience, the situation wouldn't be so bad.
The affiliates are often mentioned. The jay leno show had been severely detrimental to the ratings of the late local news on NBC affiliates. Several stations across the country have seen what is known as the "Leno Effect", where the lower audience for Leno (as compared to NBC's scripted prime time offerings) is translating directly into severe audience drops for late local news (on the order of 25–30% The local news was suffering because it had to be played at 11 instead of 10. That extra hour really ticked them off.
Now, had Conan been doing well, all NBC would have to do is cut jay leno free and focus their attention on Conan. the fact that Conan lost millions in viewers meant that NBC would have to restructure their late night line up again.
Finally, Conan didn't want to move time slots. When he refused, it was inevitable that he would leave NBC.
So, is it leno's fault? partially. Nobody expected his show to be a big hit. Is it conan's fault? a little more- he didn't keep his audience. Is it NBC's fault? without a doubt. They shouldn't have mixed up the late night viewings in the first place.