Apple probably won't implement USB 3 until it is somewhat widely accepted by the industry. If their phones and devices utilize USB 3, but other devices lag behind, it leaves Apple to be the forerunner with a non-utilized piece of hardware in their devices.
When laptops and desktops are running all of their ports as USB 3, then Apple will probably jump in. Besides, as the industry goes to the new 3.0 ports, Apple could buy millions of the older 2.0 ports at a killer savings. This allows them to refresh their Macs without raising prices. Even though the port probably costs less than a penny per unit in bulk.
It's all about the profit for Apple, most of the time.