There are many different types of Chihuahua, and the ones who have curly tails are also known as long-haired Chihuahuas.
Although there is no official answer to the question, several long-haired Chihuahuas were bred specifically because they had that unique coat type. The reason for this phenomenon may be that their genetic lineage includes other breeds like the Poodle or American Curly Coat Retriever because it’s primarily in these kinds of breeds where you see curlier hair growth patterns…