The Babylonian tradition involved only one light. The tradition of gradually increasing or decreasing the number of candles lit in the course of the eight days of Hanukkah, ascribed to the schools of Hillel and Shammai, is a custom foreign to the Babylonian Talmud. The latter integrates it by portraying it as an act of exceptional piety.
The sources do not reveal the motivation behind the evolution in Jewish practice, and we would not expect them to, but it is, nevertheless, illustrative to recall the manner in which the Babylonian rabbis have moulded the Palestinian Hanukkah tradition, and how the result has brought their praxis closer to the Zoroastrian customs. In the Babylonian tradition, with its “miracle of the cruse of oil”, we see a shift from the repair of a sacrificial altar or, alternatively, a tool for illumination to a focus on the illumination. The impurity of the receptacle has been replaced by concern for the impurity of the fuel. The detailed ritualization of the kindling of the light—one light—elevated, sanctified, guarded, evokes the Zoroastrian fire cult. In sum, the focus of the holiday celebration has shifted from the synagogue to the domestic candle.
Geoffrey Herman, “Religious Transformation Between East and West: Hanukkah in the Babylonian Talmud and Zoroastrianism”, in Religions and Trade: Religious Formation, Transformation and Cross-Cultural Exchange between East and West, ed. Peter Wick and Volker Rabens (Leiden & Boston: Brill, 2014), 272-273.