In Jain dharma, onions are not eaten mainly because of ahimsa (non‑violence) and samyama (self‑restraint). In simple words:

• Root vegetable = more himsa: Onion is a root (kand). To obtain it, the whole plant is uprooted. This kills the plant and disturbs countless tiny life‑forms in the soil.
• Countless micro‑lives (nigodas): Roots are considered to harbor far more minute beings; eating them leads to greater injury to jiva.
• Disturbs spiritual calm: Onion (with garlic, leeks, etc.—the “panch‑kanda”) is pungent and is traditionally said to inflame passions (kashaya), making equanimity, pratikraman, and dhyana harder.

Digambar and Shvetambar views:

• Both traditions traditionally avoid onions (and other roots) for the above reasons. Laypeople observe this as an anuvrat; monks and nuns keep it as a mahavrat with even stricter care.

For a short, practical explanation, see this note from JainKnowledge: