Lot of pluses in a small block. Distributor's at the back for better weight distribution. Plus it's mounted vertically, so the electricity won't leak out like on the big ones, where it's all tipped over 'n' such. Parts are smaller, so they cost less. They fit in smaller cars, so you don't need nearly as big a tow rig. They're more economical, and with the price of C-12 these days, that can make a difference in the racing budget.
You know the old saying; Once you go A, you never, ummm...well, danged if I know.