Ancient Languagues and More

I was reading yesterday about Khipu, the ancient language of the Incas and a long running theme came into mind. Why is that ancient languages are so difficult to decipher? We can read the human DNA but cannot decipher the coded languages of our ancestors. Albeit most of the languages now have been deciphered using advanced mathematical modeling (Egyptian, Mayan and the last I heard Harappan too). The process of decoding asks a few questions:

1) Why the languages were are so complex in the first place. If we take so long to understand it, then how is it possible that it was a common language that every one understood and spoke?

2) If indeed the problems have arisen only because there is lack of proper documentation, what steps are necessary to ensure that our present day documentation reaches the future generations? I understand that each generation believes its form of communication to be foolproof but the actual experience may be different.

Language apart, most of the ancient civilizations seem to use complex technology ( or simple applications) and then suddenly they disappear. This trend is clearly absorbed. The Incas built Machu Pichu, a modern city on the high mountains and left ostensibly because there wasn't enough water, the Mayans built temples and observatories and left, the Harappans built a whole civilization and then abandoned it. I cannot recall more, but I remember way having read about more cultural groups of people who seemed to love the build the disappear mode. The best part is we cant even figure out why they ran away, forget decoding how and why they built the things they built.

The importance of history as a means of understanding evolution cannot be questioned. If for example our ancestors knew about an impending natural disaster and decided to abandon ship, then such tools would be use to us even now. For all our scientific advancement, some of the largest causes of death and loss of property are natural disasters. Further, if as we assume our ancestors lived in harmony with nature, we could possibly get some clues for incorporating the same in our way of life. Wishful thinking?

Possibly, but the mysteries of the unexplained seem way more interesting that the dull nature of the known.