Why is Lucretius against the traditional practice of Roman religion in On the Nature of Things?

There are two reasons Lucretius opposes traditional Roman religion. The first is that he considers it factually inaccurate as an account of the nature of the universe. Instead, Lucretius favors a more materialistic account of the universe based on physical laws.
More importantly, though, Lucretius believes that traditional Roman religion is not a good basis for creating a happy life. He believes that the best life is one in which one is untroubled by external events and enjoys peace and tranquility. He sees traditional Roman religion not as leading to peace, but rather to constant fear of offending capricious deities. Because religion exploits fear and anxiety rather than helping people escape from them, Lucretius sees it as a bad influence on society. Instead, Lucretius recommends Epicurean philosophy as a better guide to how to live one's life.