In almost all avatars it is seen that Lakshmi is the consort of the Lord. For ex: Whether be it mother Sita or Visnupriya or Padmavati (Lord Venkateswara’s consort). And not Radharani, why is it that way?

As Krishna is the original personality of Godhead, so Srimati Radharani is His hladini sakti manifestation, or His eternal consort. Since Narayana in Vaikuntha is an expansion of Krishna, similarly Lakshmi is also an expansion of Srimati Radharani. For that matter, all the goddesses of fortune in the spiritual realm are manifestations of Srimati Radharani. Having made the above statements, we can identify the particular consort of the Supreme Lord by a particular nomenclature also. For example, Sita is the consort of Lord Ram. Ramadevi (found in Sri Brahma Samhita) is the consort of Maha Vishnu. Each of the expansions of the Supreme Lord have their consorts, or energetic expansions. They go by various names, but the original Personality of Godhead is Krishna, and His eternal consort is Srimati Radharani.