The USPS sells address updates in bulk for marketing purposes, and they are federally regulated.
The USPS makes only a few million from selling this data but saves over a billion dollars by avoiding the cost of forwarding mail sent to prior addresses.
The USPS is allowed to do this under an exception for it in the Privacy Act.
If you were renting it's possible your landlord or management company provides information on new residents for 'cost saving offers'.