The Hajj is known to be the pilgrimage that male Muslims do when they want to visit Saudi Arabia, which is known to be the holiest city for all the Muslims. It seems that there are a lot of Muslims who still pay a lot of attention to gender differences. They believe that males are stronger than women.
This explains why they allow a lot of men to do religious duties more than men. They believe that only those who are considered to be the strongest should perform Hajj. They may believe that women are not strong enough to undertake this pilgrimage. A lot of people all over the world are saying that this is a sign of gender discrimination.