Why do Sunni's fast on the day of Ashura and the Shi’a don’t? Didn’t the Prophet (pbuh) fast on that day because Allah (swt) drowned the Pharaoh?
Sh'is do not accept the hadith saying to fast on Ashura as valid. They are considered fabrications which came about in/after the time of Yazid to promote Ashura as a day of celebration. (Indeed, in some areas, Ashura is still celebrated as a holiday.)
For more information, you can look at resources such as this: https://www.al-islam.org/understanding-karbala-allamah-sayyid-saeed-akhtar-rizvi/appendix-4-fasting-ashura