Can anybody tell me probable reasons for this decision
Just some guesses:
Its a great machine. replacing 727s with 757s follows the same reasons pax airlines did in the 1980s. It burns less fuel, needs less crew, is more reliable and carries 20%+ more volume.
As to why the 757 ISO other types
A300 : too big and too old.
A310 : too long range optimised
737-3/4 : too small
Where to buy them >The market for used 757s has been improbably tight recently and Fedexs announcement goes a long way to explaining why.
Fedex say they have half already under some sort of option and I would suggest looking at the purchasers of the 787 series and expect to see trade ins of their 75s for the rest.
I think this will be off to freight dogs shortly where you may get some more ideas.