Dear Rabbi I was wondering why on Shabbos and yom tov we do not say Uva letzion at Shacharis but we say at Mincha instead?

Shalom! I’m familiar with two reasons which compliment each other: 1. The Roke’ach explains (about 750 years ago) that when the morning davening is extra long, then u’va l’tzion is pushed off to mincha. The Kolbo (about 670 years ago) adds that it’s difficult in the morning for the elderly, pregnant and nursing to wait so long to hear Kiddush. Even though this prayer really isn’t so long, we should learn from here (and similar halachot, like rolling the Sefer Torah to the right place before davening) the severity of even slightly discomforting our fellow Jews ("tircha d'tzibur"). 2. The Avudarham (about 700 years ago) explains that the masses would attend the rabbi’s lesson in the afternoon before mincha, and the psukim of ge’ula and kdusha were especially fitting to end off the aggadita customarily learned at the end of the session. Even today, some have the custom of ending off such lessons with that optimistic phrase: “U’va l’tzion go’el!”.