Because as much as we love trucks, for some drivers, the enclosed cargo area of an SUV is needed, and the Expedition is built on the same chassis as the F-150, so it should be pretty straightforward to produce. And we already know the Expedition is durable.
At least, that's the thinking of the 4x4 nuts at off-road.com, where they are speculating on the possibility. The off-road.com analysis points out that Ford sells about as many ultra-high performance F-150 Raptors as Nissan sells Titan full-size trucks in total, and the Raptor's price tag ensures that these sales are extremely lucrative for Ford.
So why not expand to include the Expedition? One obstacle is the Expedition's use of a cushy-riding independent rear suspension with coil springs, while the Raptor uses a rugged solid axle suspended by leaf springs.
But the Raptor actually uses a heavily modified frame compared to that of the regular F-150, so it seems there's little reason a Expedition Raptor couldn't also get its frame customized.
It would make an awesome big brother to the upcoming revived Bronco mid-sized SUV.