The reason is quite simple: although the descendants of Emperor Pedro II are a big number, a lot of them have renounced to their rights to the Throne in order to marry a commoner or are descendants of these Princes who have renounced.
In 1908 the eldest son and heir of Princess Imperial Isabel (daughter and heir of Pedro II) renounced to his rights to marry Countess Elisabeth Dobrzensky von Dobrzenicz (they were the parents of the late Countess of Paris and of Prince Pedro Gastao, the father-in-law of the recently died Princess Patricia).
Therefore the rights passed to Isabel's second son, Luiz, and then to his own son Prince Pedro Henrique.
Pedro Henrique had 12 children, but only a few of them are still members of the Imperial Family of Brazil, because of they're unmarried (Prince Luiz Gastao, Prince Bertrand, Princess Isabel, Prince Rafael, Princess Amelia and Princess Maria Gabriela) or married equally (only Prince Antonio and Princess Leonor, married to two Princes of Ligne).
The present members of the Imperial Family are Imperial and Royal Highnesses, Princes of Brazil and of Orleans-Bragança; all the other male descendants of the last Emperor (descendants of morganatic marriages or Princes who have renounced) are Royal Highnesses and Princes of Orleans-Bragança.
The Head of the Family is only Imperial Highness and Prince of Brazil (titular Emperor of Brazil), since as Head and titular Emperor loses his rights to the French Throne.