because people inbreed them to death. That's another reason I like my common guppies so much, they tend to be the result of outcrossing strains, and they go from fancies to an almost wild form. They're a little less inbred (although all fancies tend to be too inbred) and are generally used in breeding programs to "deepen" the available gene pool.
I currently have a nice magenta and yellow male with a flag fin. He came in a big frog fodder purchase, and I really like the look of him. My other "big" male is all pastel pinks and blues, and from the same batch of breeders.
they also don't take long to recover from bad conditions at the petstores. We have a variatus (fancyxcommon) who survived 3 months in my 55 degree goldfish tank! (Mike tried feeding his excess fry to the goldfish and they missed one) She is now my son's pet fish, and her name is "Crit" after a "crit roll" for survival (D&D).