Why has vegetarianism become a Buddhist commandment?

Being a vegetarian is a sign of compassion!

When the Buddha was alive, monks begged for food and donors ate whatever they gave them. They can't ask for vegetarian food. After Buddhism was introduced into China, due to the different national conditions and climate, it gradually evolved into a temple to cook and eat at a fixed time and place, which provided convenient conditions for vegetarianism. During the Southern and Northern Dynasties, Liang Wudi, who believed in Buddhism, was influenced by the Buddhist concept of universal compassion, and strongly advocated vegetarianism, which later became a fine tradition of Buddhism in the Han Dynasty.