Attitude - Stones couldn't have found his feet back in the City back unless he had the will.When City were considering to sell him in the summer he said he wanted to fight for a place and stay at City. It was his sheer determination which helped him re-establish himself.
Personal issues - Stones had off the field issues with his ex-wife. Recently his ex-wife blatantly criticised him in a press release with the Sun.Fortunately his off the field issues seems to have been solved now as Guardiola himself said in a press conference in the recent past.
Ruben Dias - In Ruben Dias Man City have found a true leader at the back which City have been lacking since Kompany left.Dias has been evidently commanding the backline and improving Stones,Stones seems to get better and better everyday playing with Dias.
Pep Guardiola-The way Pep Guardiola eased Stones into the squad has to be commended
.He initially gave Stones opportunities against easier oppositions and dead-rubber matches in Champions League and then went onto play him week in week out ,even against tough oppositions.

Competition among CBs in the City squad was a major factor too,there are 5 CBs in the City squad competing for places.Pep Guardiola's preference of Eric Garcia over him towards the end of last season was a major reality check for him as Garcia was just an youth team player.