Upper respiratory infection, trauma, masses, neurological disease damages vocal cords damping volume.
Larynx Issues
Meow muscular coordination still developing in young kittens under 3 months old.
Kitten Development
Oriental lines like Siamese vocalize more while Persians and British Shorthairs make fewer sounds.
Breed Tendency
If every need met, cat may feel communicating through meowing serves no purpose.
Lack of Motivation
Previous howling or screaming bouts strained throat muscles interfering with later normal meowing.
Overused Voice Box
Unique personality filters communication style - talkers versus strong silent types!
Random Happenstance