I'm no DARPA-ologist, but I would hypothesise that this spec is for an initial phase to develop / demonstrate the technology, to be potentially followed by a more demanding spec aimed at a potential deployable asset.

Also, bear in mind that by far the majority of aeroplanes don't have to take off from a carrier. While being able to take off from a carrier is definitely a useful feature, being able to take off from a 1500ft runway still opens up a lot of potential places to fly from - old WWII aerodromes, for example. If you have a friendly airport nearby, there's less need to rely on using a carrier (though you'll still have to arrange fuel supply and other support stuff)