Because we are missing one of natures most powerful anti-oxidants which not only tastes good, but is a natural to enjoy as a thirst quencher through out the day. Black tea is partially crushed and fermented before it is dried - the fermentation destroys most of the catechin. My own theory is that my English ancestors were introduced only to black tea, as took months to ship it to Europe from China in sailing ships with no refrigeration. But this was not how tea had been used in Asia for thousands of years. The Chinese and Japanese only knew the fresh green dried variety and their herbalists certainly knew, and still know, of its benefits.
Catechins are the polyphenols, bioflavinoids that act as super antioxidants.
They have been found to have significantly higher antioxidant powers than grape juice and wines. These studies have not only quantified the benefits, but have started to establish which of the many polyphenols in the tea provide the body with that particular benefit. The most important catechin is Epigallocatechin gallate. Because the subject is so vast, the benefits established in these studies will be itemized under these two headings. Two detailed articles on this subject can be found in the Life Extension Foundation June 1999 edition.