I'm guessing price. The Eaton G80 LOCKER (not limited slip) has been around a while and is not cheap. I have one in my Trailblazer 4wd, in my 2003 Duramax 4wd, and now a third in my M6 Camaro.
Its available in the 1LT and above, I'm guessing as an order option unless some dealers order theirs that way. Here is an old school type video: once one wheel achieves about 100 rpm faster than another a flyweight mechanism engages and locks both sides together.