The correct answer to this open question is the following.
The principle of training that would best explain why a Major league baseball pitcher may not necessarily be an elite soccer player is the principle of Individualization, which states that an athlete has specific skills and abilities to perform in a particular sport or athletic ability.
Every sport demands specific requirements and athletic abilities to perform at the best level. Athletes born with specific qualities that could be improved to training, coaching, and teaching, but in certain fields of sports. That is why some athletes have the skills to be very good in some athletic activity but not in other sports.
The other principles in sports regarding training are the principle of specificity, overload, periodization, and progression.