In Jain Dharma, onions are avoided because they are root vegetables and are considered to contain or foster infinite or many subtle living beings. Uprooting the whole plant also causes greater himsa (violence) than picking fruits, grains, or leaves.

Jain practice avoids:

• Root vegetables like onion, garlic, potato, carrot, beetroot, etc.
• Foods that increase attachment, passion, and bodily stimulation.

Onion is also traditionally avoided because it is believed to:

• disturb mental calmness,
• increase tamas or heaviness,
• and is not suitable for a pure, restrained Jain diet.

So, the main reason is ahiṁsā and purity of conduct.