Why shouldn’t onion and garlic eaten?
There is a mythological story behind this. During samundra manthan when Vishnu who had taken form of Mohini was distributing nectar to the demigods a demon also came there. Thinking him to be a demigod, Lord Vishnu gave him some nectar. Then the Sun and Moon demigods told Lord Vishnu that he is a demon. Immediately Lord Vishnu beheaded him. But since Rahu the demon had taken the nectar in his mouth he became immortal.
When Lord Vishnu beheaded Rahu, few drops of nectar fell on the ground from which onion and garlic grew. Since these two vegetables were grown from nectar they were capable of curing diseases and killing germs.
Since the nectar dropped from a demon’s mouth these vegetables have a strong odour and they are inauspicious to be offered to the Lord. Also whoever consumes them their bodies become strong as the demons but mind becomes polluted and develop negative thoughts.
Whoever consumes onion and garlic their mind and body develop mode of ignorance. One is not able to concentrate spiritually. That is the reason why scriptures don’t allow consumption of onion and garlic.
Hare Krishna 🙏