Short bodies are in fashion at the moment. I have no idea why, since they're one of the worst strains that have been developed IMO... the fish can't swim, in the case of bettas they can't mate, and the compression of the internal organs (particularly the swim bladder) results in major problems as they get older. Just like other animals, it's a case of fashion over commonsense or the welfare of the animal.
Although all dogs are developed from the wolf, the healthier breeds today have a much larger gene pool. The problem with the breeds where health problems are very bad come about because they are so inbred - some of these breeds can be traced back to a single animal, and in order to 'set' whatever trait was found so special about this original animal, brother-sister and parent-puppy matings were used for several generations. Unfortunately, the betta hobby is starting to suffer from the same genetic bottleneck, and also the infestation of rosetail genes, as people breed for bigger fish with bigger fins without a thought for the fish. The entire halfmoon strain traces back to a single fish, and in the halfmoon especially we are starting to see this sort of rubbish cropping up.