Foreign residents by definition are not citizens. They don't get to vote in elections of parliementary regimes either. But assuming foreign residents could vote, then anyone can come en masse and just change the outcome of the vote. As for slaves, they weren't citizens either, but either mostly war captives which were taken into slavery rather then executed or citizens which were stripped of their citizenship for various crimes. War captives or people who commited crimes don't get to vote in elections either in parliamentary regimes.