How come Betty and Barney Rubble never had their own kid in the Flintstones?

Bam-Bam Rubble isn't the Rubbles' natural biological son-he was adopted. Fred and Wilma Flintstone having a baby was a major event. How come they didn't do that with the Rubbles? Was it because they were side characters, and making them have their own kid detract too much from the show?