So why is this considered a prequel?

Umm, perhaps you mean sequel instead of prequel?

Zathura is considered by many to be a sequel to Jumanji because Zathura was a sequel to Jumanji in author Chris Van Allsburg's books on which the two films are based.

In fact, in the books, the Zathura game was discovered to be on the same game board as Jumanji, just on the other side.

That being said, Zathura screenwriter David Koepp was insistent that the movie not be considered a sequel to Jumanji, so he wrote the screenplay so that the connection to Jumanji not be shown. That is why you were probably unaware of the connection.