+1. The industry wide trend for routing cable guides under the bottom bracket started in the mid-1980s, though cases can be found as far back as the early 1970s boom. It appears to have originated with high, Italian bicycles and trickled down. The area under the shell is more open, making it easier to thread the cables, at least for the rear derailleur, which is the cable most often replaced.

Originally, the guides were metal troughs or tunnels, brazed onto the shell. The plastic guides are simply a more economical solution, though they do have the advantage of providing less cable drag than a painted metal guide.