The fact that the M-14 was designed in 1954 and the BM59 was not known to the US until 1958-59?
Also, after the FAL was almost selected and the Infantry Rifle service trials were rigged against it to favor the US rifle in 1953-54, there’s no way in hell they were going to allow some “damn Eye-talyan” rifle design in - especially not as they’d have to pay a license fee to Beretta and the FAL design was being offered for free.