Because the properties seem like they'll only make you any money after you own them for a year real time. That one's particularly frustrating because if you don't do a mission a week there you get nothing. Go deliver some drugs at the icons n you'll get
you don't make money from all properties, just public service properties like bars and clubs. you wont get anything back from the docks, helipad, or airports other than the obvious ability to house boats, choppers, and planes respectively.