Because langauge acquisition is a very different process from learning a langauge. Langauge is based on symbols...in sign they are visual symbols, in oral language they are spoken sounds which represent the thing. In order for those symbols to have meaning, they must be understood as being representative of the thing they are meant to represent. In other words, unless one understands that the sound made when saying the word "chair" actually represents the object, then the sound is meaningless for understanding and communication. It has no more meaning than a grunt.
Deaf infants from birth are unable to receive and process the stimuli which allows them to attach meaning to the sounds of spoken language. However, they are able to attach meaning to that which they can perceive visually. They are not hampered in their acquisiton of a visual language, so that the meaning attached to the symbol of a sign is something that occurs naturally through development and peripheral exposure.
I understand what you are saying here. However, I don't see how you couldn't say that CS could do the same thing. If the child sees the cues, and they are related to a visual object such as a chair, wouldn't the same process occur?
CS represents the phonemes and morphemes of spoken language. The gestures used do not represent an object or a concept in the way that a sign and a word represent an object or a concept. Understanding the phoneme does not translate to understanding the concept represented by the symbol. Phonemes in and of themselves are meaningless.