Mainly it was moved because the "forehead" area is very busy. There's the driver ledge (with the glass-mounted chip that drives the display), two cameras, the earpiece (complete with hearing aid compatibility, which means a bigger coil), antennas for gps, wifi, bluetooth and cellular (secondary TX, because transmitting out the bottom is better for SAR as it's further from your head), proximity sensor, several microphones and loads of flexes.

Much easier to have it down in the main antenna area where the main occupants are antennas, menu/touch button, lightning connector, speaker+acoustic volume and a mic. Less flexes, more room in general.