Tennis players are very resistant to change by nature. As you can see by some of the major tournaments such as Wimbledon. The scoring system was devised in the 1800's and no better solution has ever been invented.
The tennis scoring system dates back to an old form of the game called Real Tennis. It orginated in British public schools and the scoring system is in reference to the fact that the different lines and areas had different points alloted to them. The scoring is just a hangover from this.
I don't know why but it can be about the popularity of tennis between high-society people. Maybe it has been invented by some royals to make slaves or others unable to understand counting score and playing the game.