It is only part of the explanation though; Soul/Spirit and spiritual Power, not willpower, as well as Fate has a large role in Middle-earth.
Consider if you will the essay about the nature of Glorfindel, he is said after his death and resurrection to have been much strenghtened by his self-sacrifice and subsequent stay in Valinor, on a spiritual level, and to be almost the equal of a Maia; on a spiritual level. This spirituality and soul and fate permeates Middle-earth, not just will, and it seems clear to me that in order to perform "magic" you need both willpower and spiritual strength; that one is moot without the other. I would also question Sam's ability to defeat Sauron; defy the temptation of the Ring is not the same as defeat Sauron in a battle of will. It is "simply" that; defying a temptation. And Sam does it not as much by willpower, but more by spiritual strength that comes from his simple, unsophisticated purity of spirit and devotion to his Master, his "grounding" in the simple soil of the Shire.