What I see that's interesting on my Marti is a "Retail Order"
According to Marti a Retail is – an individual comes to a dealership and orders an as yet unbuilt vehicle.
Why did it take 10 months to sell?
Left factory 12-13-71
Sold 10-20-72
On occasion, a customer will make a specific order, then "piss back" (the official term in the Ford dealership I worked at) and the dealer is then stuck with the unit. That probably isn't too bad unless the customer ordered some hideous color combo or very undesirable combination of options. I have also seen a sales managers order some really odd stuff that didn't sell. Sometimes the regional sales managers will push some really obnoxious combinations. Yours doesn't seem to fit that description, but I mention this for the sake of discussion.
I have run into the "Lot Queen" issue a lot studying the build sheets of other makes, where a vehicle sat on the lot for a long period. Usually, they were small dealers with an incompetent sales staff. Back when it was allowed (after about '67 it wasn't) you could take a leftover vehicle and retitle it for the next year. I ran across a Jeep pickup that had been built in late 1963 as a '64. When it didn't sell in '64, in October it was retitled as a '65 and it was finally sold in mid-1965. Close to two years on the lot. I have seen many such issues in the two makes I have studied the most, Jeep and Scout (be kind to the competing makes, gentlemen ( : < ).
If you can research the dealer it was sold from, you might find it was a small and/or that was poorly placed to sell Broncos... like mid-town Manhattan.