Cellular phone providers do not offer six-month contracts. The standard contract term is 2 years. The cost of the handset is the reason for the contract length.
When new customers sign up for a cellular service, the actual device is generally offered at a deep discount, as long as they sign a 2-year contract. Without a contract, the handset costs the regular price. The idea behind the contract is twofold. Not only does it help guarantee a customer for at least 2 years, but over the course of the contract term, the handset is paid off via normal billing. Those wishing to purchase a phone at full price can often get no-contract or month-to-month service.