On IOS 2
When the 2.0 firmware was released, framework changes were present. All of the applications compiled for iPhone OS 1.2 and below were no longer able to run on 2.0. Therefore, Installer 3 wasn't available for 2.0 and the vast majority of users switched to Cydia. At that time, RiPDev (Russian iPhone Development) took over the Installer project and started developing Installer 4.0. When Installer 4.0 was finished, it was added to the QuickPwn software as a payload. Installer 4.0 did not gain as much popularity as Installer 3.0 because most of the jailbreak users had become accustomed to using Cydia.
Read more about this topic: Installer.app