The difference is that the Spanish and Portuguese were Roman Catholic. Roman Catholicism has always lent itself well to religious syncretism. Catholicism itself consciously employed syncretic techniques throughout its early history in an effort to win converts.

Catholicism, with its panoply of saints, its rituals and iconography was an excellent match and cover for slaves continuing to practice their traditional religions. Not so easy to practice a pantheistic religion within the confines of Protestantism. The Afro-syncretic religions are mixtures of various traditions, but the Yoruba pantheon is the predominate one found in these practices.

It wasn't just the Spanish and Portuguese either. Remember, the French were also Roman Catholic. So you'll also find syncretic survivals in the Voodoo of Louisiana and the Vodun of Haiti, both of which honor the Yoruba pantheon.

http://www.historytoday.com/matt-salusbury/did-romans-invent-christmas

http://www6.miami.edu/iccas/afro2.pdf

http://www.newadvent.org/cathen/14383c.htm

http://www.atitlan.net/maya/mayan-religion.htm

I think its a sort of things. But most important: were landed more africans in South America (Brasil indeed) than North America. For example: Salvador, Bahia's capital, has the largest black population outside Africa. Naturally, with a greater number is less difficult to preserve their culture.

Some comparasion in America slavery http://www.digitalhistory.uh.edu/disp_textbook.cfm?smtID=2&psid=3044

Well known fact (from http://en.wikipedia.org/wiki/Salvador,_Bahia): The enslaved were forced to convert to Roman Catholicism, but their original religion, Candomblé, has survived in spite of prohibitions and persecutions. The enslaved Africans managed to preserve their religion by attributing the names and characteristics of their Candomblé deities to Catholic saints with similar qualities. Still today all Candomble sessions are conducted in Yoruba, not Portuguese.

Related questions:

Were most slaves in North America taken from the Caribbean instead of directly from Africa?

How common was the smuggling of slaves to Brazil after the British abolished slavery but before Brazil did?