Some reasons for this can include:
Genetics: Siamese cats are known for having a mutation in a specific gene that affects the production of an enzyme called tyrosinase. Tyrosinase is involved in the production of melanin, which gives hair, skin, and eyes their color. In Siamese cats, the mutation in this gene results in the production of less melanin, which leads to the characteristic blue-point coloration and reduced melanin in the inner ear, which may affect hearing and vocalization. It is speculated that this genetic mutation could also be linked to increased vocalization.
Communication: Siamese cats are very social and communicative creatures, and they often use vocalizations to express themselves. They might meow to get your attention, to ask for food or treats, or to greet you when you come home. While most cats communicate with a soft gentle purr, Siamese cats have a much more vocal way of communicating.
Environmental factors: Siamese cats can also become loud or vocal if they are bored, stressed, or anxious. If your Siamese cat is suddenly vocalizing more than usual, it's important to try to identify and address the underlying cause.