Nominative is one of the four grammatical cases of German.
Cases show the roles of various parts of a sentence.
English has two cases in pronouns -- for example, we say "I see him and he sees me." and we cannot say "I see he and him sees me".
The German nominative case is used, for example, for the subject of a sentence -- much like the English subjective case in words such as "I, he".
So the reason for why welche Getränke sind... can't be anything else is similar to how "they are ..." can't be anything else; you can't say "them are ...", for example, because it's just the wrong form to use for that role in a sentence (subject).