Ford's 4.6 liter V8 and 4r70W transmission are bulletproof and reliable. The body-on-frame design is built to withstand taxi and police use. Plus, the whole car is designed and marketed to previous generations that value durability, quality and reliability -- rather than younger generations that value Bluetooth and touchscreens.
I have a family of six, and in a pinch this car can carry all of us.
First, many original owners of the Grand Marquis take fastidious care of their car, and drive very few miles. Second, since most buyers my age and younger are simply not interested in a V8, body-on-frame land-yacht, there is a ready supply of clean, well-maintained, low-mileage cars for less money than a newer, higher-mileage car with more "toys" and less build quality.