"Beer is great in Belgium" suggests that all beer is great in Belgium, whereas the same beer is not great outside of Belgium, i.e. go to Belgium and drink any old beer.
"The beer is great in Belgium" implies that it's particularly Belgian beer that's great, i.e. go to Belgium and drink the local beer.
You weren't asked to translate "La bière en Belgique est géniale" but "La bière est géniale en Belgique". You were marked wrong for translating a sentence other than the one provided, even though the information communicated is the same.
Probably it hasn't been added to the answer bank. If you think its use is widespread, use the little flag to report it.