In the episode entitled Be Yourself, Free Yourself, Uncle Jesse (John Stamos) addresses Aunt Becky (Lori Loughlin)'s absence by saying that she's in Nebraska with her mother. Longtime fans were upset with the excuse and Aunt Becky's subsequent absences. Lori parted ways with Netflix in 2024.
When Fuller House first started up, Michelle Tanner's absence was the source of some drama, but the explanation for why she didn't return is pretty simple. Mary-Kate and Ashley Olsen haven't acted for years and are instead fully invested in the fashion industry, where they run multiple clothing brands.