Yup, fitness, genetics and too many variables.
As I wrote in post
#45 , multi-champ Manny Pacquiao has experienced intermittent leg cramps for years. Team Sky's 2015 budget was estimated at just under $40 million. Pacquiao grossed $160 million against Mayweather alone.
If cramps could be cured with "fitness", pickle juice, Gatorade and $10 worth of potassium and magnesium supplements, it's a safe bet one of the highest paid and best trained athletes in history would have figured it out already.
Training to or beyond exhaustion to cure cramps is like the old high school football coach mantra to push exhausted teenagers beyond exhaustion with two a days in summer, no water allowed. It "cured" weakness by killing players. It defies recent years of scientific, methodical research. Practice doesn't make perfect. Perfect practice makes perfect. Improper training only leads to injury and further debilitation. Once muscles are exhausted and cramping we lose coordination and resort to poor skeletal alignment that destroys joints.
"There is always an easy solution to every problem - neat, plausible, and wrong." - H. L. Mencken