Does "of" always follow the word "popularity"?

Hello,

My English teacher says that after "popularity" we must always choose "of" so he chose "of" in this sentence:

- Nobody knows the real reason for the film's popularity.....viewers.
the choices were: (with - to - for - of)

My question is: Popularity of viewers? is there a popularity of viewers? I don't understand that sentence, is it some term I just never heard of? shouldn't it be popularity of the film *to* viewers?

He insists it is always "of" after "popularity" and says that it is even a sentence in the main textbook, but the sentence in the textbook says:
-Nobody knows the real reason for the popularity *of* the film.
Here, it makes sense. but up there I just can't process it, so can someone please explain it for me?

Thanks in advance.