^ What he said. Although it doesn't really make sense since all the F series up through the F450 get the full power version and the gcvwr of the E series is much lower than many available F series class 3 and 4 trucks.
Similarly, the E series, cutaways, Class A chassis and bread trucks also only come with a 6 speed.
Moreso, I would bet it comes down much less to the durability and duty cycle rating and is primarily a cost saving measure. Which is somewhat easily justified in that the "performance" (power and speed) of a utility vehicle is much less of a selling point to plumbers and RVers than it is to F series buyers, who actually tow much heavier with class 3-4 trucks.
And with no real competition to beat (other than Chevy who isn't even close in market share and also not winning the power/transmission battle anyway), no reason for extra cost to give someone a faster class C or work van.
In other words, Ford is not chasing a carrot with these models. They are the carrot, albeit a lower HP de-rated carrot.