Why does the Qutub minar not rusted stil 

Iron pillar of Qutub Minar is not rusted because it was made by 98% wrought iron. Presence of high amounts of phosphorus (as much as 1 per cent against less than 0.05 per cent in today’s iron) and absence of sulphur/magnesium in the iron are the main reasons for its longevity.A thin layer of a˜misawitea (formed catalytically by the presence of phosphorous in the iron), a compound of iron, oxygen and hydrogen, has also protected the pillar. 
Because of these reasons, rusting of this iron pillar is too slow and it will take thousands of years to get rusted.