But it did!
Prior to Bane, there were as many Sith as there are Jedi. However, due to the inherent nature of the dark side, they would continually weaken their order. Groups of weaker Sith would team up to bring down stronger ones and then the process would repeat itself over and over. What you got was a dilution of the Sith order, where their numbers were many but their individual power level was continually diminishing from generation to generation.
Enter the rule of two: "Two there should be; no more, no less. One to embody the power, the other to crave it." Now the apprentice must grow more powerful than the master, kill him/her and take on an apprentice of his/her own. If the apprentice fails, he or she is struck down and a new apprentice is found. Thus each subsequent Sith generation gets stronger and stronger; a concentration of power. That's the theory anyways.
So it is not really a case of backstabbing. The apprentice is expected and indeed trained to eventually their shot at the master.