New arrivals have no incumbent to displace
Winning a customer away from a provider they have used for a decade is slow and expensive. Winning a household that has been in the county for three weeks and has no provider at all is neither.
A market with constant arrivals hands you a steady supply of that second kind of customer. Most local businesses never plan around it because it is invisible in a monthly lead report.
Newcomers decide in bundles
Someone settling into a new area chooses a long list of providers in a short window. Dentist, mechanic, gym, contractor, insurance, and the rest often get decided within a couple of months.
Being visible during that window is worth far more than being visible at a random moment, because you are competing for a decision that is actively open rather than one that was settled years ago.
The competition is running flat budgets
Most local advertisers spend roughly the same amount every month regardless of who is arriving. That means the moments of highest opportunity are not the moments of highest competition.
Concentrating spend into those windows buys more attention for the same annual budget, which is the closest thing to free money this market offers.