“Belange von Minderheiten” – why “von” and not a genitive?

Answer 1

Depending on where it appeared, most likely it is the author or speaker trying to avoid the Genitiv, which is falling out of popularity / use more and more. One could also make it a compound noun, "Minderheitenbelange" and get around the Genitiv / Dativ problem..

Answer 2

There are two reasons why:

(1) There is no plural indefinite article that could be changed into genitive; instead "von" (here: meaning "of") is used to express the possessive relationship.

(2) Nobody in Southern Germany really cares for genitive and very often dative is used instead. "Von" is a dative preposition meaning "of", and in Southern Germany, it could be used instead of genitive as well.

Hence, both (1) and (2) lead to the same result.