The newer engines have been proven to be much more dependable so they do not require 2.
Just quoting someone else who explained it to me - The competition really made/pushed P&W step up their game. The engine before that added pressure was injected into them is LEAPS behind where the current one is sitting at. The project litteraly went from "just another contract" to "OK boys, our balls, reputation, the kush contract and our !@#$ is now on the block", internal resources and assets went to unlimited to beat and top what GE was comming out with.
GE saw this and was still trying to secure the contract so they also stepped up their game to match, and that is what really touched off the war in congress - you had two mega defence contractors going balls-to-the-wall in R&D of these engines for a period of time that was vastly being funded purely on the companies own dime up-front and then turning around to the government for compensation. Both wanted to at least secure the promise of being recompensated up to date if not full funding for the complete development and ultimate production. Finally, after numerous extensions, they said enough, and so if GE wanted to continue forward internaly with it they had no promise of seeing another dime from the government for it.
While I think the GE engine shouldn't of been nixed and may of even been better, and that book may well be written by now, it has done its job in arguabley the good/best ol' fashion competitive American way... the bean counters will never forget the numbers involved, but it will/has payed off.