What makes leaving business cards in mailboxes effective for marketing?

It is not effective, and it is not legal. In the United States, mailboxes are federal property reserved for stamped mail, so slipping a card inside can draw fines and almost always ends with the card in the trash. The better use of your budget is a distribution method that actually reaches people.

• Direct mail: a USPS-compliant postcard or mailer is trackable and lawful.
• In-person handoffs: networking events, trade shows, and counter drops build real recall.
• Community placement: bulletin boards, local shops, and door hangers on the door, not in the box.

Spend your effort on channels that convert and keep your reputation intact rather than risking a penalty for a card no one keeps.