Symbian open source never took off
I spent many hours dabbling in, or at least attempting to dabble in, Symbian.
Yes, in theory it was open source in that you could download the code, well most of it, and build it. But in practice this was not effective.
It only built on Windows (yes there was a sub-project to try to also support Linux building).
Building was very arcane and needed two or three different toolchains that worked in different ways.
There was no real online community.
The only people that ever really got engaged in this were phone vendors. General hackers, the real backbone of open source, might have sniffed the tyres but then walked away and went back to Linux, *BSD, whatever where it is far easier to get involved and be productive.
Basically, nobody at Symbian really seemed to take on board that there is more to open source than just making a server public. Effective open source requires input and mentoring.