The soils in many countries may be woefully deficient in Magnesium. Therefore it simply does not enter the food chain in adequate amounts for ongoing health.
Any use of energy also uses large amounts of Magnesium.
High doses of magnesium taken orally may become problematic for the GI tract. Basically, they can upset your digestive system causing nausea and diarrhoea.
So transdermal or topical magnesium avoid the digestive system completely, thus avoiding the digestive problems. In the words of M. Sircus:
"Traditional methods of administering medicine such as tablets or capsules get watered down and become much less effective due to stomach acids and digestive enzymes, before they eventually get into the bloodstream. Bypassing the stomach and liver means a much greater percentage of the active ingredient goes straight into the bloodstream where it's needed.
The full potential for trans-dermal medicine has not been explored by modern medicine though it has been practiced for thousands of years in hot springs around the world." (i)