Comment by inkyoto

I can't answer the why question, but out of all Germanic languages, only Faroese has retained the plural indefinite article, although its use is restricted to nouns that 1) only occur in the plural and 2) indicate «a pair/set of», e.g.:

  einar buksur (feminine plural) – a pair of pants

  einir skógvar (masculine plural) – a pair of shoes

Other than that, Germanic languages typically do not have a plural form of the indefinite article.