The reason for the 50-move rule, along with the triple-repetition rule is to make chess definitely finite. How did one arrive at the number 50? What would change if it was 40 or 60? Is the number 50 chosen carefully?
At one time, it was believed that all winnable endgames could be won within fifty moves. However, in the early twentieth century, some exceptions were found, including A. A. Troitsky's (1866-1942) analysis of the two knights endgame as well as the endgame of a rook and bishop versus a rook. The rules of chess were revised several times to admit exceptions to the fifty-move rule for certain specific situations. Early on, the fifty-move rule applied to tournament games but not to match games (Troitzky 2006:197).
SOURCE: Fifty-move rule