It wasn't.
Cydia was the only iPhone OS 2.0 Jailbreak portal at the time because Installer's new owners were running things badly (ie 2-month late Installer 2.0) and eventually led Installer to it's death.
Also, Cydia was being worked on (since Installer's devs were busy squabling) during the first iPhone OS 2.0 betas. When iPhone OS 2 came out, Cydia was it, and many smaller devs jumped ship from Installer to Cydia.
Cydia came to be thanks to Installer. So to make it official. Installer was before Cydia and before the App Store.
Take into account, Installer 1.0 version icon looks terribly alike the current App Store icon. Also, the layout and organization was very much alike (except with fancy Apple gimmicks)