I'm not sure what you're saying... Do you want wider tires in the rear than in the front on 1/8 scale 4wd buggies, like in the 1/10 4wd ones?
Well, for one thing, 1/8 scale tires are already slighly wider than 1/10 rear ones... Maybe going wider doesn't offer much benefit?
Or maybe it's cheaper to be able to use all tires on front and rear, rather than having two different categories?
Either way, in theory, twice as much tire width gives you the same amount of traction. Tire width is not a very important factor, because the wider your tires, the more surface area your car rests on and therefore less pressure per square inch on the ground. Well, sorta. In off-road it probably matters more because with wider tires you'll be pushing more dirt behind you. I'm not really sure how much diff it makes anyway.
Just trying to guess and explain the phenomena... I really don't know why it's done like that (all tire widths are the same in 1/8 scale 4wd cars).