If one considers the evolution of the pipe it is logical to conclude that the earliest of bits (such as they were) were of the orific design because it was the simplest to produce, being merely an extension of the stems' round lumen. It is further logical to conclude that this was continued when (according to Alfred Dunhill in his "The Pipe Book") ebonite began to be used in 1878 by an English manufacturer as a, "...really cheap yet efficient mouthpiece..."
I daresay that as manufacturing technology improved post 1878, coupled with the desire of pipe manufacturers to improve their products and, hence, their market shares, the flatter wedge-style bit was a natural next step. Easier to clench, it offered the added benefit of widening the smoke stream (imagine one's tongue being hit by a stream of water from a hose nozzle in the "jet" setting versus one in the "fan" setting, and the difference becomes easy to visualize).