Why are the tenets of Tae Kwon Do in Chinese when written out rather than Korean?

Tenet = An opinion, doctrine, or principle held as being true by a person or especially by an organization.

First, note that what we know today as the Korean writing system was not created until the mid-15th century. The first tenets of Tae Kwon Do, however, were established in 7th century Korea. Korea, at this point in history, was still using the Chinese system of writing.