>>27428794A number of reasons probably.
1- Ash's defeat with Shota resulted into somewhat of a downward spiral for him which culminated with the Snowbelle incident.
2- Considering the previous point Ash managed to turn the tide to his favor, to which some extent Serena also had a part, even if little on it, and is perhaps at his best after that.
3- It's Serena after all.