He never wanted to be in One direction. He was forced to be in it against his will.He felt that he couldn't fully be invested in the band soleheartly, since he couldn't put his input in any one direction decisions. They a dit it didn't fit the image of the band. So like honestly he wasn't doing something he was happy about doing with his talent.It was unlike him cause it didn't really pull out Zayn true talent. With One direction music. And so he left it cause he could finally be himself doing what he loves. Altho he loves the boys in One direction. It was just time to leave.
Zayn being a part of One direction basically served as a stepping rock to get to the platform he desired. And to get the experience to get to that point.