וילן שם כי בא השמש (ויצא כח:יא)

Unlike Shacharis and Mincha, which are a חובה, an obligatory prayer, Ma’ariv was established as a רשות, a voluntary prayer. Why is Ma’ariv different? Because Yaakov had initially set out to daven Mincha, as it was still daytime. But in the midst of his tefilla, the sun suddenly set, and night fell. Since his initial intention was not to institute a new tefilla of nightfall, but merely to complete Mincha, the prayer that emerged—Ma’ariv—retained the status of רשות,