How does supply drop?

There is currently 17'947'512 total bitcoin available. Out of those 17'947'512 around 3% is being traded daily. There are still 97% left in supply to be traded. Okay lets say there is still around 80% left due to the fact that lots of bitcoins are lost somewhere never to be found.

So where does this shortage of supply comes?

Why should people suddenly start buying more BTC?