Genetics is absolutely vital. Imagine a team of twenty boys doing the exact same workouts, eating the same diet, and getting the same about of sleep each night. At the end of the season, some boys will be much faster. This scenario plays out on every team in every high school in America. Some people are just much, much better. You can compensate by "working harder" but what can you do when the most talented kids are also working super hard?
But you can still be very good even if you are "normal." Try your hardest to be the best you can be (D3 was great for me), and don't worry if you aren't in the top 10% in the sport.
You asked if "someone" could explain this to you. The answer is yes - Epstein and Hutchinson.
The first book you (and all fans of the sport) should read is called The Sports Gene by David Epstein.
The second book everyone should read is Endure by Alex Hutchinson. It is just as good.