Stacey's parents' divorce is a major plot point in the books, and it's also a story that plenty of kids (and adults) can relate to. Dawn and Kristy are both children of divorce, but their parents' splits happen before the start of the show. Seeing Stacey actually deal with her parents' divorce in real time would be a powerful storyline that could also help young viewers navigate the emotional toll of a split by opening up a dialogue about the complex emotions and major decisions (like which parent to live with) that children deal with when parents decide to end their marriage.
From a character standpoint, introducing the divorce storyline could also bring Kristy, Dawn, and Stacey closer. It only makes sense that Kristy and Dawn would rally around Stacey, which in turn would challenge the established best friend pairings of Mary Anne and Kristy (plus Mary and Dawn), and Claudia and Stacey.