I developed 3 apps on J2ME at that time and investigated Symbian. The impression I got of Symbian of that time was that it was painful to develop against. J2ME was less painful but very, very limited. Think no animation support, just bitmaps, and I have to write my own game run loop, collision detection, etc.... This was most probably due to the limited hardware of the time but nonetheless, it was primitive.
In comparison, iOS and to a lesser extent Android are basically fully featured OSs with mostly full exposed functionality. iOS gives you core graphics, opengl es, core text, core animation, grand central dispatch, networking, etc... Android gives you views, animation, background tasks, activities, intents, etc...
The distribution model was also broken. If you wanted something distributed, you had to basically go with a carrier. There were attempts at an app store but there were not successful.
Contrast this with the App Store, Google Play, and iOS enterprise development program where you don't have to go begging to a carrier to get your app distributed.
To summarize, Symbian was coming from a previous era and had no chance. If you compare RIM's situation to Nokia's, I bet you will see a similar chart.