Many Jains (especially more strict families and ascetics) avoid jackfruit, but it is not as universally known a rule as, for example, “no root vegetables.” The main reasons given are all based on ahimsā (non‑violence):

1. Many seeds = many potential lives

- One jackfruit contains a large number of big seeds. - Each seed is considered a separate one‑sensed living being (jīv). - Eating the fruit usually means destroying many such potential lives at once, which increases hiṁsā (violence) and karmic bondage.

1. Presence of many small organisms

- The sticky, fibrous flesh of jackfruit often contains very small insects and many microscopic organisms. - While cutting, cleaning and eating, countless tiny lives may be destroyed without us noticing.

1. Higher overall “life‑density” (anantkāy‑like concern)

- Jain ethics encourages choosing foods that involve minimum destruction of life. - Dry grains, pulses, and simple fruits with fewer seeds are preferred over foods where many lives are clustered together. - Jackfruit, with dense, moist, sticky pulp and many seeds, is therefore seen by many as a higher‑hiṁsā food and avoided.

1. Traditional practice and guru‑guidance

- In some Jain communities and under some ācāryas, jackfruit is clearly listed among items to be avoided. - Householders trying to follow stricter Jain diet rules often continue this traditional restriction out of respect for śāstra and guru‑āgñā.

1. Variation among Jains

- Not all Jain families strictly avoid jackfruit; practice can differ by region, sect, and how strictly one follows dietary vows. - But no Jain tradition recommends jackfruit as an ideal or preferred food; if it is avoided, it is always on the side of caution to reduce hiṁsā.

In simple words: Jains avoid jackfruit because eating it can involve destroying many lives (seeds and small organisms) at once, which goes against the principle of ahimsā and careful eating.

You can read a clear explanation here