The right being acquired by being in the locality. Homeless people can and do vote.
One can argue it is difficult to verify someone is telling the truth about residency or argue whether six months is a good choice of time period - those are valid implementation questions, but I strongly believe that in a just system, you need to be there.
Suppose a locality is bitterly divided over an issue that affects everyone, and elections centered on that topic come down to the wire. In my county, that issue is upzoning.
If you suppose that elections centered on this issue come down to the wire, it strikes me as unjust to run elections under a system where votes are up for sale, and the outcome of the election can be swung by people who have spent no time in the locality.