Business Strategy
Apple did not put the A6 chip on the iPod Touch 5th generation for the following reasons (at least my speculation):
1. Maintain its pricing structure for the iPod Touch - the A6 chip is the fastest chip that Apple is currently producing. If Apple placed the A6 chip on the iPod Touch 5th generation, the pricing structure would have to change - higher prices would have to be charged for the iPod Touch. Apple does not want to lose the main market that purchases its iPod Touch. Thus, putting the A5 chip on the iPod Touch 5th generation makes more sense.
2. Controlling the limited supply of the A6 chip - since the A6 chip is fairly new and there is limited supply of this chip (at least when the iPhone 5 was launched), Apple wanted to make sure that it used that A6 chip on the product that would net it the most profit - the iPhone 5. Thus, the A6 chip went on the iPhone 5, a product that caters to a different market than the iPod Touch.
3. Keeping a smooth sales flow of its products - after the A6 chip matures and supply grows, Apple will begin putting that chip into its other devices like the iPod Touch, iPad, and mini iPad; consumers will be happy and will be looking forward to the next best thing from Apple. By this time, Apple will be more than half way done working on its next chip (A7?). Thus, the cycle will begin again. The A7 will go into the iPhone (Apple's flagship product), then gradually move onto the iPads and iPod Touch.