Pboddie has a great post above and I really think he nails it with:
As to why Symbian wasn't attractive as an open source project, perhaps details like the licence and project governance, plus purely practical matters around actually being able to compile the code and deploy it on something, might have had something to do with it. Throwing stuff over the wall doesn't build a community, nor does choosing a licence that isn't compatible with various other widespread licences, even though they didn't do a Sun or Microsoft and write yet another one.