I wrote to you in the earlier post about certain difficulties with breeding goldfish. To successfully breed goldfish, one would need a large, pond like area, and Platytudes is right-- you would end up with thousands of eggs and quite a few spawn, some of which would be downright unattractive. Everybody has to start somewhere in the breeding trade, but personally, I am chicken and don't have the space, and I have been raising goldfish for 20 years. It takes some good experience to end up with a high quality product. Less important if you are breeding pond fish for non professionals. More important if you are breeding fancies.
If you're asking why I like cories, you probably know yourself, since you have some They're really, really fun to watch and the breed very well without a lot of fussing and intervention.