What is the difference between "finnes det" and "det er"?
"finnes det" literally translates to "does there exist" while "er det" translates to "is there". They are often interchangeable in Norwegian.
While realising it meant "exist" by circumlocution, I thought literally it was "does there find itself".
Why aren't they interchangable in this situation?
Well, yes, but it would sound just as awkward as "There exist many bars in downtown".
English speakers will not get the meaning of 'Downtown' unless they are bilingual in English and american (english)
It's certainly not British English but Brits are familiar enough with the term from American films and TV shows. I wouldn't see it as an issue.