Pretty much what people have said: It's not that we don't like goldies, it's that they're more difficult (expensive) to keep (simply due to the size of tank they need), particularly if you're wanting to keep a lot of fish or a wide range of fish.
They
are very dirty fish compared to most fish of their size (so are plecos, and most members will say so, and yet many members keep them), and it's important that people know that they need more space than most tropicals of their size.
I love my koi, and I loved my goldie. When I have the chance to build a pond, I'll be stocking it with a few common
goldfish and one or two more butterflies.