We also have to remember that all these choices were made before they were sold off. It wouldn't surprise me those people who made those choices are gone almost every time. Management changed every time Motorola was sold.
Motorola is using more than one plant, but those plants are not the size of the US plant nor has the capacity to build millions of units per week.
I feel Google underestimated demand, and with the Lenovo purchase being approved it's up to lenovo to ramp up production. If Google had done things soon or if the Lenovo deal didn't go through yet maybe Google may have gotten 2-3 more plants to have them built. Possibly reopen the US plant just for the Nexus 6.