In Jainism, onion is avoided because it is a root vegetable.

Main reasons:

• Ahimsa: When uprooting onion, many tiny living beings in the soil may be harmed.
• More injury to life: Root vegetables are believed to involve greater violence than fruits, grains, or above-ground vegetables.
• Spiritual discipline: Onion is also considered to increase rajas and tamas tendencies, which can disturb calmness, self-control, and purity of mind.

So, Jains avoid onion and garlic mainly to follow non-violence, self-restraint, and a pure, disciplined way of living.