Because MAP, Michigan Assembly Plant, is tooled for the latest Ecoboost engines called Nano series. The 2.3L, 2.7L, & 3.0L are the Nano series family of engines. To accommodate the 3.5L would have required plant tooling changes and shutdown. By staying within the Nano family all the various vehicle configurations may proceed down the assembly line at any time.
Same situation for our Fenders, they are as wide they could be to feasible fit thru the MAP paint booth. Ford would have rather had larger painted fenders but that would have required plant shutdown and millions in updating the paint booth.