Design practicality.
As all current 1/8th buggies are mid-motored, shaft driven machines, the design lends itself very easily to using a 3rd diff. Having a 3rd diff helps take some of the shock loadings off the transmission, too.
On the much lighter, usually belt driven 1/10th cars, the extra rotating mass - plus the fact that integrating a 3rd diff wouldn't actually be easy to do in the first place - means that we race 4wd electric buggies with only 2 diffs. Slipper clutches reduce the shock loadings, so electric buggies still enjoy reliable transmissions. Most people agree that 1/10th buggies actually handle better than 1/8th buggies.