How did "Checkpoint Charlie" become known as the border crossing between East and West Berlin? Out of the countless border checkpoints, what set Checkpoint Charlie apart from the rest for defectors?
There weren't "countless" border crossings between East and West. The checkpoints Alpha, Bravo and Charlie were the designated checkpoints for use by allied forces personnel (there were others which only West Berlin citizens could use).
Alpha and Bravo are less famous because Alpha was the main crossing for the "inner German border" dividing East and West Germany proper, while Bravo was in the south-west of Berlin. Alpha and Bravo were used for transiting between West Germany and West Berlin (crossing East German territory but nothing to do with East Berlin). So these crossing points were rather mundane compared to Charlie which was in the heart of the city and was used to transit between East and West Berlin.