So famous and so well known? That is an achievement, to be sure...
It was the first. It was the best marketed, and offered the most prize money. It is in the middle of the season - the Vuelta is the newest and least remunerative, and had to change from spring to autumn because it was competing for riders with the Giro. It now competes for riders with the world championships. As a result it is definitely third in the GT rankings.
Very few riders have the capacity to win both Giro and Tour in a single season, so it is in the interests of the team sponsors that the top riders are seen in the most prestigious and best-publicised race. So success breeds success, the Tour gets the strongest fields, etc. etc.