Why is Birkat Kohanim only done on the 3 festivals and Yom Kippur outside Israel, when in Israel it is done every week, and in some places every day? Surely, like all Mitzvot that don’t relate directly to the land of Israel should be practiced equally in all places of the world (like Trumot, Maaserot and Shmita are kept only in Israel because they only apply in israel)?

The standard custom in Israel and the standard worldwide Sephardi custom is for the Cohanim to bless Birkat Cohanim daily during Shacharit. The standard Ashkenazi custom outside of Israel is to practice Birkat Cohanim only during Mussaf of the festivals. The reason is brought by the Rema (OC 128:44) to be that since in order to be able to bless, the Cohen needs to be in a glad-hearted joyous state of mind, and that is not possible but on Yom Tov where we are commanded to rejoice and set aside the troubles of livelihood and Parnassa. Even on Yom Tov it will be only during Mussaf since it is just before the festive meal.