Paul Manafort lives in the same Trump Tower as Donald and family. He has known Donald and the family for more than 30 years and is very close to them. He is in effect part of the extended family. He is wealthy and retired, did not in any way need the job.
His role was to shore up delegates in the face of the nevertrumper efforts. Now that he has fulfilled his mission. he wants to go back to being retired and following his own pursuits.
Manafort did exactly what Trump hired him to do, to keep his delegates committed through the voting process at the convention and deliver the nomination. And now that that job is done, Manafort's expertise and forte with primary delegates is essentially over. This is not a surprise.
Great job Paul Manafort. Good luck in the future.