Our selection of weapons is based on what weapons are noteworthy and superior to others in their own respect.
The M60 has been and still is a substandard GPMG. Mediocre at best(M60E3). The fact of the matter is, why should we implement something of the lowest common denominator when there are better options?
Nostalgia? I'm sure many people who've used the M60 in vietnam must have loved it. The fact you needed asbestos gloves to change the barrel not to mention that while the barrel is being changed you lose the support of the bipod. Or perhaps the fact that the cheekrest puts the user's face so high up that he can't make any use of cover? Or the propensity for the rear leaf sight to go inexplicably out of alignment?
Heck, the only reason the SEALS use the M60E3 is it's the smallest MG in that caliber.
No offence mate, but the M60 really is pretty crap when you compare it to other designs of the same caliber.