This makes sense as manufacturers do not apply everything the first year out. They leave room for future advances to keep up with compitition.
Power increases cost many bucks in warranty repairs as components get more and more stress. The diesel truck industry is so close and competitive that manufactures only sell what is needed to stay with the pack.
We all know the 6.7 has a lot more potential than Ford has given it but the costs would be a real burden if they sold the 6.7 at max output. They will increase the power as needed and affordable for them.
It all comes down to "power costs money - how much do you want"? If we ran our engines at full output we would be spending mega bucks and loads of time in the shop. Of course, the time spent using that power would be a lot of fun!!
I'm sure Ford will pump the 6.7 up at some time in the future, and hopefully they will make that upgrade available to the earlier trucks, but they need to see how much warranty expense each power level costs them first.
Also, you can bet that GM and Dodge both have some 6.7s in thier shops being maxed out to find out what they are up against for future developement of thier products. That's how it works IMO.