Is it the main purpose of law to promote morality?

No, not at all. Morality isn't something to be legislated. The goal and focus of law should be strictly about keeping a functional and peaceful society. Not dictating morals. There may be some crossover where functional societal behaviours are also inline with a moral behaviour but not concretely. Morals are too subjective to be rationally and " morally " legislated and enforcing laws decided purely on a morality basis is tyrannical and unable to be enforced without yeah handed and dangerous authoritarianism.