When Android overtook Symbian, Symbian was a more capable OS than Android, this had nothing to do with capability. Android was able to copy iOS faster.
Sure it was more capable: it'd been around a lot longer and had a mature ecosystem. However, it also had fewer constraints on where it could go and what it could ultimately do, which Android (and iOS) weren't saddled with. That started to get better from Symbian^1 onward, but by then Symbian had essentially lost. The S60/MOAP/UIQ platform fragmentation before that didn't exactly help.