al_burpe
Observer
I would guess it's just that as decent as the 3rd gen Expedition is, it doesn't have any feature that distinguishes it over the much more common and much easier to modify Chevy/GM platforms. The problematic 5.4 on the early models probably keeps some folks away and I'm sure IRS does too.
I don't know what the raw sales numbers are for the Expedition vs. the Suburban/Tahoe/Yukon/Yukon XL/Escalades are but from my seat-of-the-pants observation, the numbers seem to be about 5:1 in favor of the GM products. So it's not just that "overlanders" shun the Expedition, Ford itself seems to have, at best, a half-hearted interest in them. Given that Ford seems to make a dizzying array of SUVs it's not surprising that the big Expedition gets lost in the shuffle.
Really, for the question "why don't we see more Expedition builds" you could substitute "2008+ Sequoia" or "Nissan Armada" just as well - all are decent full sized platforms that market to the Soccer mom base, but are virtually unheard of on these pages.
I tried to look up sales numbers as I thought you were way off. I could only find for one quarter of 2018 but you were just about right on. It didn't have the navigator or Cadillac sales but they probably cancel each other out. I agree that up until the current generation that Ford didn't seem to put much effort in. Why did they keep putting the 5.4 in the Expedition long after they started putting the 5.0 in the F-150?
I would say though that the 2007+ Tahoe and Yukon are just about a sarce on here as the Expedition and other bof vehicles you mentioned even though they retain the solid rear axle.