The reason it is called iOS 2 is because it was the second major release of Apple's mobile operating system, originally known as iPhone OS. It succeeded iPhone OS 1 and introduced significant new features, including the App Store which allowed official third-party applications on the iPhone for the first time. The naming follows a straightforward version sequence, with "2" indicating the second main version release after the original 1.0 version, which was simply called iPhone OS 1 or iPhone OS. This version came out alongside the iPhone 3G in 2008 and marked a big advancement in the ecosystem by promoting third-party app development and other enhancements.