Why are hollow steel are more stronger than solid steel?

You have to be carefull with blanket statements like that since there are plenty of situations (in tension for instance) where it simply isn't true. However, a tubular section will be more rigid than a solid rod of equal cross sectional area. This is because of the increased external dimensions of the tube. This means that a given angle of flex requires a larger strain: the gerater external dimensions the tube to stretch more than the solid rod. That additional stretching requires more force in order to achive it.