P: 5,634
Likely no real reason anymore...just a classification...an historical artifact like so many other things

The Wikipedia description of fundamental focres is typical:

http://en.wikipedia.org/wiki/Fundamental_forces

 ...Gravitation is by far the weakest of the four interactions. Hence it is always ignored when doing particle physics....
but this is not very accurate.... they should say "everyday" or 'typical' particle physics.
In studying high energy collisions for example,such as at CERN, gravity isn't usually simportant....BUT

'particle physics' in neutron stars and black holes, for example, and with electron, neutron, and quark degeneracies gravity is EVERYTHING...