Because "sein/his" is a possessive pronoun describing the accusative noun "Bier." Since "Bier" is neuter, you use the accusative form of "sein" which is just "sein." "Wir/we" is the nominative pronoun.
Although the genitive case is a type of "possession", possessive pronouns are actually an adjective which can describe nouns in all cases. For instance, "Meine Tasse ist klein", "Tasse" is the nominative noun and is feminine, so "mein" uses the feminine nominative form which is "meine." You would only use the genitive case of a possessive pronoun if it was describing a noun in the genitive case.
This sentence along with this one seem to both have non-functioning audio. As far as I know, it seems to only be a problem for these two particular sentences.
Not to mention that this sentence often appears as a "type what you hear" question; only making it all the more difficult. Anyone experiencing similar issues?, or know what's going on?