Have you actually used it or developed anything for Symbian?
It was neither "open" nor "for developers". Some of the sources were available, but unlike Android, you couldn't rebuild the OS from scratch and load onto your device.
You couldn't just build and run your own apps. You had to obtain certificates from phone manufacturer (Nokia) and sign them. Thankfully the security system had so many holes it wasn't a big obstacle.
J2ME was a second-class citizen, only to support a lot of existing J2ME apps from feature phones.
Qt was way too heavy for the devices of that time. Qt apps easily consumed 40-50 MB of 128 MB RAM, while C++ ones that used native APIs could fit in 5-10 MB. But the native API sucked. It was in a bespoke dialect of C++, riddled with a lot of generational legacy. "A lot of other frameworks" you mention were mostly stuff used by some single developer for their own apps, to workaround all the Symbian quirks.
Idk, I don't understand people trying to whitewash Symbian. It didn't die because of someone turning evil, Microsoft acquisitions, whatever. Ultimately it died because it was unacceptably bad for a modern phone OS. Been there, used it, don't want to, good riddance.
MeeGo, though, is an entirely different beast...