sima :
Why this illogical compromise?
I don't know. That's just what the datasheet says.
sima :
Are there other ARM Cortex M3 MCUs available that supports simultaneous CAN and USB?
It is likely. You're on your own for that research, but you could start by looking at every vendor that has Cortex M3 products. It's a short list, but promises to get longer as more adopt.