Because of the serious nature of Kaliyan and the boons claimed by him. So the power of the 'Supreme Ekam' was given birth as son of Lord Narayana in the Name of Vaikundar. Hence Vaikundar is not only a unique avatar but also the only able power to destroy the evil Kali.
Due to the severe nature of Kaliyan and the benefits he promised, the power of the 'Supreme Ekam' emerged as the son of Lord Narayana in the form of Vaikundar. Consequently, Vaikundar is not only a singular avatar but also the sole force capable of defeating the malevolent Kali.