Vietnam was a very different battle field than the middle east. In the ME, you have longer engagement distances with 300 to 600 yards not being uncommon and marksmen/snipers being able to extend that range to 1,000 yards.
7.62 works well in that space. M-14 is a rifle already in stock, so it is a natural fit. But truth be told, any 7.62 rifle that is reliable would work.
Alternatives include:
M1 / M14 in bullpup configuration
AR10 and variants like the Remington R25
Keltec RFB
FNH FNAR
FN SCAR
FN FAL
{and others}
You forgot the Mosin Nagant.
CC
You forgot the Mosin Nagant.
CC
The M14 is not for gurls.
While I very much enjoy FALs, they are problematic due to the issues with mounting modern optics, which are de rigueur these days for the longer engagement distances given as the driver for this decision.