Of course its COST...it's always COST.
But there's more to that statement.

Cost VS Benefit/need

Say Ford has tested DI on these engines (including the 5L which was rumoured to have it from the get go about 2 years ago)
And it got MINIMAL gain, but costed 'x' number of dollars more.
A ) is it worth the extra coin B ) will the customer care C ) what is the cost comparison between the 2 if they do offer it, and is it REALLY worth it.

You as a customer, how much would YOU pay for minimal increase?
Would a customer be willing to pay an extra 3-5 thousand dollars for 5hp?, 10hp?, 15hp? What are they willing to pay for.
And can the engine PI or DI meet the goals set forth? (400 was the mark, and they obviously hit it without DI easily)

Remember the 5L won out because of COST....lower cost VS other configurations.
The 5L family has more in it, this is not the most the engines can do.
But without seeing the test numbers of PI VS DI, on the SAME engine, we really can't comment on what we think the increases will be.

Bottom line, they hit the target, they'll save money doing so.
So yes, it was cost.

 

I would think that the engineers start out with a list of technologies, some high tech like DI and Ti-VCT, others lower tech like tuned tubular exhaust headers. You figure out how much each thing would cost to implement, you test to see how much gain (in efficiency and/or power) you would realize, and you choose those things that have the greatest cost/benefit ratio. In this case DI isn't worth the money. On another engine maybe it is. In the future, maybe it the cost of the components will come down enough to make sense on the Coyote.

 

Remember, a lot of the gain from DI is in raising the compression ratio to take advantage of the cooling effect. Also, in the 5.0 MM&FF article, Ford engineering claims it got some of the benefits of that cooling effect by firing the injectors while the intake valve is open. All in all, its maybey 15-20 more horsepower left on the table, and some money in your wallet.