Why is the scoring in tennis structured as 15, 30, 40 instead of a more linear progression?

The scoring in tennis is structured as 15, 30, 40 instead of a linear progression because it originated from a clock face, where the hand moved from 0 to 15 to 30 to 45 (which was later changed to 40 for simplicity). This traditional scoring system has been retained for historical reasons and to maintain the unique heritage of the sport.