I'm not sure if I misunderstood your comment. But as far as I know most iOS users install apps using the app store built into the device. It's really not much different, if not more polished than the cydia store.

I think the real reason for the popularity of cydia is that 1) it has a user friendly UI 2) you can get apps on it that will not make it to the app store and 3) some people are just too cheap to pay for some apps and use it to pirate them