Why is 60 Senate votes the magic number for the filibuster advocates?

The filibuster became popular in the 1850's as a means to stop a vote on a bill with endless debates. In 1917 the Senate adopted rule 22 which allowed for a 2/3 majority vote to end debate. In 1917 we had 48 states so the magic number to end a filibuster, or for cloture was 64, in 1975 the number was reduced to 60%, which happens to be 60.