Probably for the same reason that Ford stopped making them. Most people these days want SUVs.

GM downsized to Chevrolet, GMC, Buick and Cadillac after the 2009 bailout. They're chasing the highest profit margin vehicles, which are SUVs these days. GM, Ford and Chrysler haven't made a good profit on sedans for years and they dropped them. In Ford's case, I don't think their best selling car in the United States has been a sedan since the early 90s Taurus. That was the #1 seller at one point. Gotta find older GM cars if you want a sedan, or get a Camry/Lexus, unfortunately. Domestic manufacturers can't/ won't compete with Toyota unless there's a huge profit margin to be had. High gas prices may quell the demand for SUVs and push the economy car back to life, but that won't happen unless they take a huge hit in their sales.

Adding to what Chuck and Justin said... Buick is going full electric by 2030, and the majority of EV manufacturers are, not surprisingly, tilting towards SUVs.