It's because it was then Father Hidalgo made the proclamation. It is similar to April 19th and July 5th around here. Both are independence days but the latter gets the cake since it's the declaration.
El Grito de Dolores is the start of the fight for Mexican independence from Spain. The revolution comes way after when the fight against The Porfiriato started.
Ha, ha, ha, ha. 4 days of party sounds like the Latin way to party.