It's less expensive to mail out the forms, and use the mail to remind folks they are coming and should have been there, than it is to send census takers door to door to fill out forms on the spot with everyone.
Fortunately a large percentage of the people do fill out the forms, mail them in, and they never get bothered by a census worker. A smaller percentage are trying for cheap dates or something, eh, and they waited until the census sent someone to visit them personally.
It would be far more economical to contract the census work.