Why did the pagans celebrate mardi gras?

Technically speaking, pagans do not practice Mardi Gras. Mardi Gras is French for Fat Tuesday which is a Catholic holiday. However, practitioners of Santeria and Voodoo in the Americas blend Catholicism, Native American and African religious beliefs and practices, and as these religions all blend together in Latin and South America, some pagans probably do celebrate Marti Gras (especially in areas like Brazil) because it's more of a cultural/social thing, not really treated as a religious holiday by the mainstream.