The reason could be that some scholars do not believe the Holy Prophet (s) had more than one daughter. According to our tradition, he had only one daughter (Fatima (as)). The other daughters according to some traditions were the Children of Khala (Khadija’s Sister). They came to stay with the Holy Prophet (s) when they lost their mother.
Another reason could be if we do believe that the Prophet (s) had more than one daughter, would be that they were not at the same status as Fatima (as). Similarly, many of the Imams had many sons but we normally only hear and know of the ones who were destined for Imamate.