Why did the Calvinist gentry convert to Catholicism from the end of the 16th century?

A few reasons:
1. The ban in 1632 on the construction of new Protestant churches, the ban from 1668 on the transition to Protestantism and Calvinism. Transfer of churches to Catholics.
2. Persecution by Catholics and pogroms. In 1639 the Calvinists left Vilna. In 1668, a law was passed threatening to confiscate property from those who did not convert to Catholicism. Pogrom in 1682 – destroyed houses and churches. The voivode died of surprise.
3. The growth in the number of Catholic monasteries in the 17th-18th centuries, the active work of the Jesuit order. Opening of collegiums, some of which became universities (Vilna).
4. Restrictions on rights. For example, the law of 1733 actually equated them with Jews.
5. The activity of the Catholic Church in the framework of the pan-European process of counter-reformation (since the 1570s).
6. Conversion to Catholicism of noble cities. For example, from the Radziwill family.