Jains don’t eat potato, onion and garlic mainly for two reasons:

1. Ahimsa (non‑violence) – they are root vegetables (kand‑mool)

- Potato, onion and garlic all grow under the ground. - To take them out, you must uproot the whole plant, so the main plant dies. - In Jain view, the roots and the surrounding soil contain countless tiny living beings (nigod / sukhshma jiv). - When we pull out and cut these roots, many lives are destroyed at once, so the himsā (violence) is much more than in plucking fruits or leaves where the plant can still live. - Because Jains try to minimize killing even of the smallest life forms, they avoid such vegetables. ( jainknowledge.com)

1. They disturb spiritual calm (especially onion & garlic)

- Onion and garlic are part of the “panch‑kand / panch‑shalyā” (five pungent roots). - They have a strong, sharp smell and taste, and are believed to excite passions and restlessness, making the mind less peaceful and less fit for sādhana, samayik, pratikraman, meditation. ( jainknowledge.com) - Jain food ideal is simple, non‑violent, and supporting a calm mind. So such items are left out.

Both Digambar and Shwetambar traditions

• Both agree on the basic principle: avoid root vegetables to practice higher ahimsa.
• Monks and nuns of both traditions strictly avoid all roots.
• Most serious lay Jains also avoid potato, onion, garlic and similar roots in daily food, though day‑to‑day strictness can vary by family and region. ( jainknowledge.com)

You can read a simple explanation in detail here:

• Why no potatoes, onions and garlic