Why did Mary give Saint Dominic the rosary?

Mary did not give the rosary to Dominic as it had already been around for many years. However, St. Dominic did vigorously promote the rosary.

Saint Dominic (1170-1221) was a Spanish priest and founder of the Dominican Order, or Order of Preachers. The rosary has always been strongly associated with the Dominicans.

According to popular legend, Dominic was so anguished because of the almost complete failure of his efforts to convert the Cathars to Catholicism by peaceful means, that he took himself into a forest and fasted and flagellated himself to the point that he began to hallucinate. An apparition of Mary appeared to him and gave him the rosary. Another legend holds that the Virgin Mary appeared to Saint Dominic in the church of Prouille and gave the rosary to him.

The practice of praying Our Father and Hail Mary on beads is a practice that predates Saint Dominic, so whether Mary really did give Dominic the rosary, as we now know it, or whether it is merely the end result of centuries of evolution is a matter of debate.