Mostly looks, but also some durability considerations. The wider planks weigh more, and so the fasteners are loaded more, and the planks themselves are stressed more. Larger planks are more likely to warp or delaminate or crack, and it becomes much more visible when anything happens to them. It's the same with wood, the wider the planks, the more warpage and shrinkage and cupping and other defects show up.
Wider planks are lower cost, as there's a set 1 1/4" of waste for the overlap regardless of planks size. So, a 4" reveal has twice the waste per square foot that an 8" reveal has. Wider planks are also less labor to install, as they go up twice as fast for the same cutting, fastening, and placement labor. They're just heavier. They also take twice the number of nails, twice the number of flashing sheets. Nails and flashing sheets probably added $250 to the cost going from 4" to 8".
I just put 4" reveal on my house and garage, I wanted the classic look of the narrow, and the extra cost was worth it to me. I paid about $1.50 per square foot for the siding itself, and close to the same for labor. It would have been a bit cheaper if I'd gone with wider reveal. I'd guess overall, I paid about $3000 premium for the narrower siding, on a $20,000 job.