Why are the spoilers used as the backup controls instead of ailerons on the Boeing 777?
The B777 is a high subsonic aeroplane, and at high airspeeds the ailerons remain stationary because wing twist may cause aileron reversal.

By using inboard spoilers as mechanical backup roll control surfaces, there is roll control available at high airspeeds as well. Roll authority would be limited at approach speeds due to the smaller moment arm than the aileron provide and due to negative lift on one side only.