Why iPhone apps gets pushed to background after sometime?
You can only prevent this on a jailbroken phone. Apple trusts its apps not to hog the battery and are OS-level apps so they can run in the background. All other apps from the AppStore are not first class citizens. When they go in the background, their state is saved and restored when they are opened again. This is to prevent them from constantly updating content and draining your battery.

Long story short, you can only get that sort of control when you jailbreak. Note: if you are not careful and run many programs in the background after jailbreaking, your battry life will be really poor.