Because, The Magnesium is formed a protective layer of oxide. That\’s the reson we cleaned the Mg strip with the sand paper.
Mg+o2 give rise to magnesium oxide(Mgo) which is unreative
When magnesium ribbon is placed outside for a period of time it reacts with oxygen to form a layer of magnesium oxide, this layer prevents magnesium from burning. Thus, it is cleaned with a sand paper before burning.
Because it form a layer when it is in air and react with it . this layer prevent magnesium from burning. Thus it is necessary to clean it before burning
Because magnesium react with oxygen present in the atmosphere to form a protective layer