It's zoning ordinances and like any zoning ordinance its aimed at controlling or maintaining the character of the neighborhood, preventing frat houses and group homes in certain neighborhoods, etc. If parking for one family is expected and 8 students take up the whole block, etc. It's limited to unrelated persons because "family status" is a protected class under housing law.

Different houses on the same street can be zoned differently. My house is zoned "single family" but two houses down is zoned "boarding house."