GM had invested half a billion dollars in Lyft and had off-lease cars it was forced to sell at auction. The deal came together on a Christmas Eve call between Lyft's CEO and GM's CTO, giving Lyft a lever Uber lacked.
Durant built GM by buying Buick and Cadillac and offering cars at several price levels. When the Model T tapered off after Ford held over 50% of the market, GM's wider lineup took over and it became one of the largest companies on the planet.