Because GE and Rolls Royce have pulled the plug on their joint F136 engine venture:

After 15 years of ups and downs, General Electric and Rolls-Royce have accepted defeat in their battle to power the F-35 Joint Strike Fighter and will today announce the end of the F136 program and the Fighter Engine Team partnership.

By deciding to discontinue self-funding the F136 alternate engine they are also parting ways at an interesting time for the propulsion world. Rolls’s recent, and unexpected, rapprochement with Pratt & Whitney over commercial engines could, for instance, signal the start of new strategic links in the military engine world now that the long-lived experiment with GE is over.

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The move to kill the F136 comes after an Oct 31 meeting between GE Aviation leadership and Deputy Secretary of Defense Dr. Ashton Carter in which “it became clear that the DoD would not support the FET self-funding effort,” says GE.

So basically, the Pentagon left a horse’s head in their bed.

I expect that all the good news from Pratt & Whitney over the past few months, about them making noise about being ahead of schedule and budget, to end abruptly.

Without a competitor engine, we  are now going to see all the stuff that was swept under the carpet for the F135 percolating back up, and the tax payer will be on the hook for it.