It's nearly an Apple tradition at this point: whenever the company unveils its latest iPhone, older models are used to fill the free and $99 on-contract price points. But with the announcement of the iPhone 5c today — which starts at $99 —Apple's decided to discontinue the iPhone 5. It's simple, really: the 5c fills the price point that would have been occupied by a discounted iPhone 5. The ancient iPhone 4 has been discontinued as well, but, surprisingly, the company will continue to sell the nearly two-year-old iPhone 4s. It will be sold for no upfront cost on a two-year contract.
It's possible that the iPhone 5 remained too expensive a product for the company to drop the price significantly, and with the iPhone 5c there's just less space for older devices — especially since the new device is very similar to the iPhone 5 internally.