I was an insider so I was jaded and biased. But ... :)
S60 was a horrid codebase and very hard to wrangle into modern things.
UIQ was a cleaner smaller codebase where the main big apps (e.g. messaging) didn't actually use the latest greatest parts of the framework. UIQ was always being dragged in opposing directions by its two big owners - Sony Ericsson and Motorola. Sony Ericsson had some success with their early P800 and just wanted to keep on iterating that with the P900 etc, whereas Motorola wanted smarter feature phones and features like keypad navigation. Neither wanted capacitive touch, despite UIQ suggesting it umpteen times.
The Symbian codebase was an antiquated C++ dialect. I happened to have liked it, and found it very expressive and powerful and simple, but there was definitely a learning curve.
When the iphone came along it was so simple and beautiful and had a capacitive screen and basically everything else was obsolete immediately.
Android was unreleased and actually had an extensional crisis at that point, but they quickly revamped it to be more iphone-like and launched.
At that point everyone else thought "android will be cheaper to develop!" and dropped Symbian just as quick as they could.
And I don't blame them.