Why Masashi Kishimoto did not author Boruto? I mean he successfully completed Naruto, so why not continue it ?
He needed a break!

An excerpt from an interview he did at New York Comic Con (per Anime News Network):


  Christopher Butcher:
  72 volumes of Naruto. Did the editors of Shonen Jump want you to keep going? Some of the Shonen Jump series go for a hundred volumes or two
  hundred volumes. Did the editors want you to keep going on Naruto?
  
  Masashi Kishimoto: I would be lying if I said there wasn't some pressure from management, the powers that be as they were, but I had a
  pretty clear idea of how I wanted the story to come to a close, so I
  had to put my foot down and say, 'No, I'm sorry, this is it.'


It's also the case that during the creation of Naruto, Kishimoto got married and was supposed to go on his honeymoon with his wife, but never did.  After the series was completed, he was finally able to do that, even though his son was (at the time) about as old as Boruto would be in-canon.