Why are poodles curly?

Poodles have hair instead of fur causing them to be non-shedding and hypoallergenic. They have dense, curly, non-shedding hair that grows year-round and requires regular grooming. The thick coat allows for warmth to major joints when the dogs were immersed in cold water (this was what they were originally bred for - to go and grab birds shot out of the air that fell into the water) - and the coat is trimmed/shaved back in areas to lessen drag in the water. Breeders are going to select dogs with a particular coat type and breed with them, while a dog that has a thinner coat is less likely to be used in a breeding programme - the result is a breed with a thicker coat - which is more exaggerated in a show ring scenario.