Why does the appstore say that the app is incompatible with my iphone?

Okay my iPhone 3gs has an ios of 4.2.1 i downloaded apps that is compatible with that ios so why does it say that it is incompatible with my iPhone. My iPhone is up to date or updated. WhY? I downloaded the apps through iTunes and when i sync my iPhone to the computer then put the apps in it it say it is incompatible with the iPhone! Why?

Because the Developer of that App maybe made the app compatible with IOS greater than yours. For example when you are developing ios app Xcode the Software that is used let you create the apps for certain types of ios version maybe the developers created for ios 5 no ios 4.2.1 (you can always Update you iphone 3gs to 6.1 (latest ios version) we have an jailbreak and unlock (if you save you shh blooh in cydia)…

Hope it help.