Why can't I update my iPhone 4 to ios6?

Why can't I update my iphone4 to ios6?

Check your battery life. It needs to be at least 50%.

I did. Yes it needs to be plugged in or at least have 50% power

You probably have a jailbroken iphone 4.

Maybe it's the server load: There's always a high demand for the new iOS every time Apple releases it. Or…
First of all, make sure that you are connected to your local Wi-Fi network. Then, plug in your iPhone. It might take from minutes to hours to update your iPhone to iOS 6. The update should work once you do both steps.

P.S.: to @Juan Radillo, Apple had to change the dock connector in order to make the iPhone lighter and thinner. It was the only way to accomplish their goal for thinner iPhone for them. Also, Apple just has limited server speed (so does Google, YouTube, etc.) so upgrade during the high demand would take a long time to download the firmware.