Because Bethesda retconned the lore and changed it so that they had an explanation as to why their are Super Mutant behemoths in the capitol Wasteland. I do wished we had some Behemoths in New Vegas but I'll take overpowering Deathclaws instead.
There was for some reason a military vault messing around with FEV. Vaults were not for military use. Enclave would have no use for such a Vault. FEV was to only be on the West and was top secret. Bethesda went against canon by adding super mutants.
"It is there game and they can do with it what they want" does not fly with me. Fallout was interplays game and they came out with a game that many people now call the "Burned Game" because it went against canon in so many ways. A game so bad Bethesda won't touch with a ten foot pole :poke:
Lucky for Betheda that is the only thing that really breaks canon besides there being radiation all over DC after 200 years and yet little to no radication in Fallout which is just 84 years after the great war. The other many problems don't technically break canon but are very hard to explain.