It could be as simple as "wealthy areas get more county services." There are practical considerations when choosing polling places, like the availability of parking and enough space to accommodate a line, check-in tables, voting booths, and ideally separate entrance and exit doors. Public schools and county rec centers are go-to locations because the county (who administers the election) already owns them and they have the space needed. Churches are great too, but they require having an agreement with a private organization.