Extremely high rent/housing prices in the city combined with the parents (and probably the rest of the family) living in the country. They didn't want the kids to spend 3 hours on the train every day so they rent a small apartment near the school and send money each month for expenses. Parents probably also don't have jobs they can just move to the city with (own a small store? farm? small company with no branch in the city?) This isn't new. Heck, it's in half of all highschool anime considering how common it is in japan.