I think it's because developers are so familiar with the current systems they are able to really push its abilities to the max... just think what they will do with PS4/XB1 in the future...
The big difference though with BF4 is that current gen will have 24 players versus 64 on next gen. Big BF maps would benefit with the extra players IMO. Not sure when I will be getting a PS4 but I will give BF4 a miss until I do.