Because they side with Conan in this debacle:
-Jay promised Conan The Tonight Show when he stepped down.
-Jay stepped down and gave Conan the show.
-Jay's new show, which was developed to cut the cost of expensive 10 pm dramas as well as keep the ratings Jay used to bring in, failed.
-NBC decided that they would put Jay starting at 11:35, effectively destroying the original Tonight Show format.
-Conan said he would have no part in that.
-Instead of being the bigger man, Jay decided to go with it. He doesn't need the money.
-NBC now lacking a Tonight Show host decided to give it back to Jay.
-Old Jay fans are pleased as punch; everyone else is pissed. The other late programming hosts are eating this up.
-Chalk up another point into the inevitable death of NBC.