The reason for this is a bit of a mystery, but it's commonly speculated that:
* The writers wanted to focus on the dynamics of the three men raising the girls. This allowed for the show's unique humor and family-centric storyline.
* It might have been a creative choice to keep some aspects of Michelle's life ambiguous. This allowed viewers to imagine their own backstory and relationship between Pam and the rest of the family.
While Pam's absence is a recurring question for fans, the show ultimately focuses on the bonds between the Tanners and how they navigate life together as a blended family.