PySide didn't support Qt5. That's because the project was basically left for dead by Nokia and then Digia - for Qt proper, bindings will always be an afterthought.

Meanwhile, PyQt just chugged along just fine. Riverbank must keep focusing on it all the time or it will die. The incentives are very different.

PySide was stuck on Qt4 and basically stalled. PySide2 is practiaclly a new project. For Qt5 bindings, PyQt has been the only game in town for years, and Riverbank has done a damn good job of it.