They had to try and deal with all kinds of display sizes, and they had their customers such as Nokia not using parts of the Symbian o/s, from what I’ve read and what my old boss has said publicly, such parts particularly being UI-related code, look-and-feel and so forth.
Interesting fact- my friend Jane completely rewrote the core of the o/s, so ‘Symbian o/s’ is not one o/s but two; made it hard real-time capable, I think, iirc. I haven’t quizzed her about it though. That was years after I had left. She wrote a book about the new v2 o/s, which in its guts bore almost no relation to the old I suspect. I’ve forgotten what the proper code name for the new o/s was.
There’s a book about about the crash downfall of Nokia which is extremely interesting reading, need to find the reference; doesn’t talk about Symbian though. In the beginning, Symbian was owned by a consortium: Psion, Ericsson, Nokia, poss Motorola [?] (I forget) and maybe another shareholder whom I can’t remember. Later on, Nokia bought up a lot of the shares. This was all many years after I had left. I think I worked for them for less than a month, then left London and went to Skye. Before then, the division I worked for was Psion Software.