No reason at all. All of my recurve limbs are made with 1 1/2" material and finish near 1 3/8" at the wedge and 1/2" at the tips.
I have found that thicker narrow limbs give better performance...at least with my designs.
I wouldn't go to radical with the curve, as you may run into some stability issues with the narrow limbs. Tiller becomes very important.
Again, design plays a huge part, but if you get it right you will come up with a very fast and stable limb.