Because not everyone wants the latest and greatest truck.
Even though Chevy has already introduced an all-new 2019 Silverado 1500 and GMC has already revealed an all-new 2019 Sierra 1500, General Motors continues to sell the old 2018 versions of both trucks alongside the new models. The company websites even list the Silverado 1500 alongside the "All-New" Silverado and the Sierra 1500 next to the "Next Generation" Sierra 1500.
The reason for selling both trucks simultaneously is because GM knows the older trucks can still attract buyers with their lower price tag. FCA has been using this same strategy with the Ram 1500 and Dodge Grand Caravan to attract fleet buyers.
According to Automotive News, GM will eventually phase out the old Silverado and Sierra by the end of 2019. The new crew cab versions of both trucks are already in production at the company's Fort Wayne, Indiana plant followed by double cabs which begin production in October. Regular cab and crew cab versions will begin production in January 2019 at the Silao, Mexico plant.
So far, sales of the new trucks have been "going exceptionally well" according to GM CFO Dhivya Suryadevara. GM has delivered 45,000 next-gen pickups in the third quarter and plans to deliver 120,000 by the end of the year - just 10,000 off previous estimates.
GM is able to produce both generations of pickups at the same time by executing a changeover called the "Oshawa shuttle." The process involves shipping unfinished bodies from the Indiana plant up to the Canadian plant in Oshawa for final assembly. GM spokesperson Kim Carpenter says the program has been a "great success" and is "on track to build more vehicles (60,000 per year) than the original forecast." It will also "run into late 2019 based on market demand."
So if you prefer the old Silverado or Sierra it looks like you'll have most of 2019 to make a well-priced purchase.