The loss of power (not loss of propulsion) resulted in the loss of hydraulics, and thus loss of control of the ship. If they had maintained power after the first outage, they may have had time to recover...but they lost power again. As it is, they were able to slow the ship in Back Emergency, down to 1.5 knots at the time of the collision.
That makes sense if the rudder were moved hydraulically, (almost a certainty) and if the hydraulic pressure source were electric. Also highly likely that the rudder control is an electro-hydraulic servo system.
'Swelp me, I'd never want to be on a ship that had no redundancy of hydraulic pump or generator, but who knows?
From my machine tool days in the '70s I know that hydraulic servos have nasty ways of going haywire, so I'd want a manually operable backup rudder valve.
I've read that WW2 destroyers used chain falls for emergency rudder control, but it would take one helluva chain fall and a team of ten or twenty men to do it for a container ship that size.