Question

why was the decision to collectivise farms taken in former USSR

Answer 1

The Treaty on the Creation of the USSRofficially created the Union of Soviet Socialist Republics (USSR), commonly known as the Soviet Union. It de jure legalized a union of several Soviet republics and created a new centralized federal government (Congress of Soviets of the Soviet Union and Central Executive Committee of the Soviet Union(TsIK) were the legislative while Council of People's Commissars was the executive) where key functions were centralized in Moscow.

Treaty of the creation of the Union of Soviet Socialist RepublicsRussian: Договор об образовании СССР
Ukrainian: Договір про утворення СРСР