It is a very important car and one where the manufacturing and supply prowess of the big automakers is put on display. The large automakers do a lot of market research to know what people want, they don't stand up and tell people what they want they go find out. They also more importantly have the money, suppliers, and trade agreements, to pivot quickly.
Throw in the most basic fact of all, if GM wants to produce a hundred thousand of these it can in short order. Tesla cannot do that because they don't have the capacity nor the money to buy it