One of the reasons is that Apple's stuff would be lost among the dozens (if not hundreds) of devices that are introduced at CES.
No offense, but Apple rarely releases hardware that is at the cutting edge. The orignal iPhone 2G and (arguably) iPad are the only two products in recent history that were ahead of the game.
Everything else has traditionally been middling hardware Apple has built into their ecosystem. Apple relies on deep support and marketing to push their products. This message would be lost at a huge show where real hardware companies (as in companies like motorola who design & manufacture the chips, screens, and boards their products are made from) have dozens of new models and prototypes to exhibit.