Probably for the same reason that full scale trucks are avalible with locking rear differentials. Basically, so it's less likely that you'll be in a situation where you'll be hopelessly stuck, because the Savage and full scale 4WD trucks only have diffs going side to side, but not front to back (well, unless you have a transfer case with an interaxle diff, but that's another story). So, if you get opposite corner wheels or both left or both right wheels stuck, you're stuck. I see it as no real advantage on an RC though, since you can't turn it on and off, and I don't know about you, but very seldom do I drive on terrain loose enough where a locked rear diff would be beneficial.