I doubt it has anything to do with it being a good slogan, the other two options are 'true place' and 'bridge for all' which are generally shit.

When you say 'bridge for all' I either think 'people walk all over you' or 'people only use you to get somewhere else,' neither of which makes the place sound very good.

And 'true place?' Was there some issue with people thinking Busan was imaginary?

At least there's really not much to dispute about 'Busan is Good' getting straight to the point and being clear.