The answer is probably that an early foundation cat or cats for the breed were noisy and talkative resulting in his or her genes being in almost all Siamese cats. In other words the trait of being talkative and noisy has been selectively bred into the Siamese over may decades. There may also be an element of learned behavior from the intelligent Siamese. The sound they make is like that of a baby getting attention. Cats are good learners.
The question in the title begs the question as to whether the original Siamese cats from Siam in the late 19th Century were noisy and talkative. And whether this is a general, natural trait of this breed or whether the talkative aspect of this breed’s character has been artificially bred into the breed from the mid-20th century onwards.
I find it difficult to believe that the original Siamese cats in Thailand were noisy and talkative. They were pointed but probably like any other moggy in terms of the quality of their vocalisations.
Therefore it is likely that the noisiness of the Siamese has been exaggerated by cat breeders in order to distinguish this cat breed – to set it apart from others – to make the cat more marketable as many people like this trait. Siamese cats do have a harsh, demanding edge to their voices and as mentioned above this may have been learnt.
Breeders do bend over backwards to create defining and distinguishable traits in their breeds so that they stand out. Otherwise they could not exist as a separate breed. The ‘intelligence’ of the Siamese (if this is genuine) has also been bred in this breed through selective breeding over years. This is what selective breeding is all about. Most times it is concerned with appearance; body and head shape and coat type and color.
P.S. There may be a difference in vocalisations between the old-style Siamese and the modern Siamese. Is it fair to say that it is the modern, slender faced Siamese who is the talkative one? Are Applehead Siamese the same?