When they come in the appear to be fine then a few days later, sick. Raised scales, ick and body fungus. I'm pretty sure it isn't the acclimation process that gives them the bacteria.If you're asking why they get sick so easily in general, regardless of if they're given good care, the answer is several reasons. For very cheap goldfish, it's because of the conditions they're raised in. They are bred en mass with poor water quality and poor nutrition. When an animal has a bad start in life, it can effect them long term. For fancy strains.... well, look at the body shape of a common goldfish and then look at the body shape of a fancy goldfish. Fancy goldfish are like the Pugs of the fish world. It's no wonder why fancy goldfish are so prone to swim bladder problems. In both types, lack of concern for genetic vitality has taken its toll over the generations.
Don't take this to mean that I hate Goldfish! Some of them are stunning, and I love their derpy personalities. It's just an unfortunate fact that they are one of the species of fish that we've messed up.