The main button was on the front, so they integrated the fingerprint reader into it. Moving the main button to the side or back might have worked but would have

The main button was on the front, so they integrated the fingerprint reader into it. Moving the main button to the side or back might have worked but would have been non-intuitive for many users (e.g. my mom just realized her iPhone 11 that she got launch day has a mute switch, and that she has had it engaged for weeks). A back fingerprint sensor would also require some gestures to change, such as swipe to unlock - attempting a complex motion while also attempting to reposition your grip to hit a sensor on the back is going to increase drop rate. They got rid of the button and the fingerprint sensor in one go, so there was never an opportunity to have just a fingerprint sensor on the back.