The mechanism at work here is production - taking 120 aircraft out of 2013-17 buys (2015-2019 deliveries). A US buy of 30-some per year only includes a handful of Cs, and although the rate goes up after the 2015 buy year, the Navy may not have enough jets for a sustainable squadron and training unit until 2019, regardless of how well or otherwise testing goes.
And pre-2019 deliveries will be off a low-rate line and eyewateringly expensive.
Not to mention the fact that the blighters are having trouble making that hook at the back catch the piece of string across the deck, which for some reason they consider frightfully important.