To try and answer the OP's question, why the manual safety demo?
As a current 787 driver I can think of a couple of possibilities, either:
(a) The IFE system was playing up and refusing to play the pre-recorded video.
or (more likely)
(b) The APU was unserviceable, requiring a ground power start. Like most systems on the 787, engine start uses electric power, and - funny old thing - it imposes quite a big drain on the ground power unit(s). As a result the aircraft cunningly load-sheds electric systems, so a GPU start is likely to knock out the IFE - hence the manual demo.