Les Williams writes:
“Why Bristols (city)and not London or Cardiff eg Perhaps Bristol planes?”
The slang is Bristol Cities, and this comes from the football team. I’m talking about Bristol City here, not Bristol Rovers, right? That’s the main reason.
Also there is a theory that it caught on because “Bristol” and “Breast” are alliterative, as opposed to eg “Leicester City”. Wouldn’t be quite the same really would it?