The phone will likely get taller. As a result, it actually gets harder to press the sleep button. It's more user friendly to have it on the sides, because that's more easily accessible for your finger or thumb.
Have you ever noticed that rotation lock / mute + volume buttons are on the right on an iPad, yet they are on the left on the iPhone? There's a reason for that.
Namely, first and foremost, most people are right-handed. In other words: most people will hold their phone with their right. On the iPhone, that means your hand will wrap around the phone from right to left: on the left side, your fingers from your right hand, can easily control the buttons.
On the iPad, your right hand doesn't go around the iPad (unless you've got gigantic hands). Your right hand usually stays on the right, your left on the left. That's why, on the iPad, the rotation lock/mute + volume buttons are on the right... so you can control it with your right hand.
A lot of research goes into small details like this. The research from Apple most likely came to the conclusion: a button on the top is too difficult to access. Instead, a button on the right will be more easily accessible for a [right-handed] user.