Why is it not A?

It is not A because there exists the possibility that someone is neither a Montague nor a Capulet. Our opening statement says "Some people are Montagues/Some people are Capulets." This means that neither family is all-encompassing. It does not say "Some people are Montagues/all remaining people are Capulets." Therefor, the possibility of a person not being either family exists. We do not know the status of these remaining people and therefor A is a could be true statement while we need a must be true statement.