Why is Dwadashi Shradh performed for sanyasins?

A sanyasin, having renounced household life, no longer belongs to the regular system of family tithis. The Dharmashastra assigns Dwadashi — the day following the supreme Vaishnava observance of Ekadashi — as the most appropriate Shradh tithi for renunciants. This recognizes their spiritual status and their transcendence of ordinary householder cycles.