F-150 is getting a diesel for 2018.
And the F-150 is a much higher volume vehicle than anyone should expect the Ranger to be. The percentage of diesel sales on the F-150 is more likely to justify the development and certification costs of a diesel. My guess is the only reason they made that decision was to compete with Ram for the full-size fuel mileage title and not because they needed the extra sales volume.
Toyota does not offer a diesel in the Tacoma and they are the current mid-size class sales leader.
Just about everyone who offers diesel powertrains is facing some sort of legal issue whether it be from the EPA or class action law suits.
I personally have a hard time seeing any advantages of a diesel in this size truck in the North American market and especially not when you have a comparable, if not superior, option with EcoBoost.
Diesel makes perfect sense if you have a constant need to tow or haul heavy loads and do it more than 25,000 miles per year. That's what they make the F-series Super Duty and 6.7L Powerstroke diesels for. Totally different market.