Why was 50 chosen for the 50-move rule? 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