If one only thinks about it in terms of costs and the business-centric arguments of open source, then the management is likely to wonder why they aren't able to fire all their staff and have volunteers develop the code for nothing ("By the next financial quarter!"). And many organisations just don't understand the openness aspects of open source (which more closely match the Free Software ideology), at best wondering why anyone would value openness (hint: it's about trust and sustainability), and at worst accusing anyone advocating openness of "wanting to give our technology away to our competitors" (followed by cries of "Nobody wants to write software for our platform!" when they close it all up).
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.