The 777 has been a much better performing aircraft than FDX imagined when they bought it. It can carry a max payload with range (which the 744 or 748 cannot) and most imprtantly, it can be done at a lower fuel burn, much lower. The jumbo, of which I have many years of loving memory, is a different animal with a different mission. FDX looked at the jumbo and decided the 777 better fit the business model, and they were right.
FDX does nothing that will not maiximize the bottom line.