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Packaging. Expense. Direct injection diesels have incredibly high rail and injector pressures (29-40k psi), and that requires a lot of high-pressure lines and tres expensive injectors. Then you have to figure on the turbos. Variable Geometry turbos used to cost an arm and a leg, but now they cost just a leg, so it's more worth it to have a single VG Turbo instead of sequential like you'd use on a V6.
Then there are the combustion chambers. It's easier to design direct injection and package it with a big chamber, so fewer cylinders still work better (up to a point)... but the 3.2 is beyond the size where that becomes a consideration, and the Ranger engine bay is long enough to take the five pot longitudinally.
As to why not sell it in the US... Allch Chcar is spot on the money. They've got a ton of money sunk into Ecoboost, and doing anything else dilutes the brand strategy and marketing push, and simply parasites sales away from Ecoboost trucks.
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