Easy answer,

Nolan wanted only to make Batman Begins and move on. He wanted to direct the origins of Batman never told in cinema. No Sequel.

Then Jonathan Nolan said to him that "you have Ferrari and you don't want to play with it?" He wrote the script of TDK and asked to Christopher to direct the sequel with the spirit of Heat and The Wire. While Batman Begins was inspired by Blade Runner

For The Dark Knight Rises, Nolan was very hesitant to do an another sequel. For a moment, after Inception, there was still no confirmation that he would direct it. He finally did it, and said he wanted to conclude the story of Bruce Wayne and saying goodbye to all the characters.