It is not understood because people’s heads are filled with 10,000 trivia and rationalizations, leaving no space for anything else.
It is not practiced because people are kept busy, though bored with the 10,000 corruptions and miseries that leave no time for the Three Treasures.
The Great Integrity is so ancient, as old as the universe itself! How can we expect people to remember it after so many millenia of repression?
That is why sages dress in rags while they wear the Three Treasures deep inside their hearts.
Tao Te Ching, Verse 70. Written by Lao Tzu, translated by Ralph Alan Dale.