When one performs ritualistic sacrifices as prescribed in the Vedas, one needs expert brahmanas known as yajnika-brahmanas. In Kali-yuga, however, there is a scarcity of such brahmanas. Therefore in Kaliyuga the sacrifice recommended in shastra is sankirtana-yajna (yajnaih sankirtana-prayair yajanti hi sumedhasah). Instead of spending money unnecessarily on performing yajnas impossible to perform in this age of Kali because of the scarcity of yajnika-brahmanas, one who is intelligent performs sankirtana-yajna. Without properly performed yajnas to satisfy the Supreme Personality of Godhead, there will be scarcity of rain (yajnad bhavati parjanyah). Therefore the performance of yajna is essential.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Ninth Canto, Chapter 4 – Text 22