Time and money are the most likely reasons why.
Cadillac's product revival is still ongoing and more new vehicles are scheduled to arrive. Eventually, the goal is to make Cadillac an EV-only brand, though this is still several years away. For now, GM's luxury division is earning bank on the same vehicle segment as everyone else: SUVs and crossovers. The all-new 2021 Cadillac Escalade premiered earlier this month. Just over a year ago, the Cadillac XT6 three-row crossover debuted in Detroit and it's now on sale nationwide. But there's a stark difference between it and its crosstown rival, the Lincoln Aviator: platform architecture.
While the rear-wheel-drive Aviator is built on a new platform designed for multiple layouts, such as all-wheel-drive and front-wheel-drive, the XT6 rides on the same FWD platform (with optional AWD) as the Cadillac XT5 and Chevrolet Blazer.
Basically, it's a stretched XT5. The Aviator, by contrast, is not a stretched version of the Lincoln Nautilus. Sources close to the XT6 project development are now claiming the original plan was to use the Omega RWD platform, which underpins the about to be discontinued Cadillac CT6. Why was this plan scrapped? The high expenses involved. Omega was designed mainly for Cadillac's use in the first place so you'd think it'd make sense for the XT6 to make further use of it.
However, reality soon set it. Cadillac realized it was missing out on the highly lucrative three-row crossover segment and something needed to come to market quickly. The XT5's platform turned out to be the fastest solution. It was probably also the cheapest.
Because this platform, codenamed C1, was already in use for no less than five other crossovers, its development costs were more than likely already paid for. Omega, on the other hand, was (and still is) brand new. Only the CT6 made use of it and getting it prepped to accommodate a three-row crossover was timely and costly.
Why would have Omega been preferred over the C1? Aside from being RWD, it has a near 50-50 weight distribution. These two elements make for a far more engaging driving experience, something Aviator owners can surely know a thing or two about.