Saltwater fish really are, with few exceptions, prettier than freshwater.
Saltwater tank stocking also appeals to a pokemon sort of mentality that a lot of people seem to have, gotta collect them all! With each tank having a requisite lion, eel, clown, damsel, hawkfish, goby, etc...
It is a real shame that many times freshwater tanks will be stocked the same way, with a cory, a pictus, a gourami, an angel, a red tail shark and 3 different tetras
The real beauty of the freshwater tank being that you can stock a true community of fish: a school of tetras, a shoal of cories, and often pairs or trios of centerpiece fish. Then it is possible to watch them interact with each other and their environment in an interesting manner that is completely impossible in saltwater without having a tank in the thousands of gallons.
So, to sum up, I think most people have only seen really crappy freshwater tanks, and assume that is what a freshwater tank is. Whereas even if they haven't seen a well put together reef tank, they probably have at least seen a salt water fish they thought was beautiful. "OOOH, a puffer!"
*Disclaimer: I clean tanks for a living and have to respond politely to 17,000 uninformed comments daily.