I think the key is the Volt was designed to be a great plug in hybrid from the start. The C-Max had an ICE only, standard hybrid, and plug-in hybrid version. Our plug-in literally had two separate high voltage batteries. Half the cargo bay was just a carpeted box full of batteries.
It was also terribly engineered. Plugging the car into the wall didn't top off the 12V battery and whatever mechanism it used to do so was inadequate so you'd go out in the morning to a stone dead car quite often. The number of recalls was insane, though we got a lot of free oil changes out of that as the dealer usually threw them in free during recall services.
First and last Ford I'll ever buy.