There could be a variety of reasons. Maybe they miss you and want to know what you're up to. Or it could be a subconscious habit. It's hard to say for sure without knowing more about your specific situation.
It could be that your ex is just randomly browsing and happened to come across your stories. Or, they might still have some lingering feelings and want to keep tabs on what you're up to.
Your ex might still watch your stories because they still have some feelings for you or are just curious to see how you're doing. It doesn't necessarily mean they want to get back together; it could just be a habit or a passing interest.
There could be several reasons. Maybe they still have feelings for you and want to know what you're up to. It might also be that they're just curious or trying to compare their life to yours.