While I can't pretend that I'm any kind of expert on fish, this thread caught my eye. I'm 38 years old and live in the UK, and so I'm part of the generation over here that was brought up with Goldfish as the standard pet. Nearly every other child I knew growing up had a tank with a couple of Goldfish in. I even remember when you used to be able to win them at funfairs or on day trips out to the seaside such as Brighton or Blackpool although I think that's no longer allowed.
I'd imagine the main reason that the Goldfish was so popular was mainly because they was easy to look after because as far as fish go, there wasn't a lot that you actually had to do. Clean the tank out and feed them, from what I can remember that's about as much as it took to look after them. Another reason I think they might have popular as a pet for children, is the fact that you could never really get all that attached to them. As a child you'd feel sad if they died, as you would with any animal, but you was never going to be distraught over something that didn't really have any kind of personality. That might sound a bit harsh, but if a animal dies and you can replace it without your child even knowing, or being able to tell the difference, then you can't exactly consider that it's become a member of the family now can you!
