Jains generally do not eat mushrooms because of the core Jain principle of ahimsā (non‑violence) and “minimum harm” in food.

In simple points:

1. Mushrooms are living beings (fungi)

- In Jain view, fungi are life‑forms, not just “flavour.” - Eating them means directly using that living organism as food.

1. They grow in impure, decaying matter

- Mushrooms usually grow on damp, decaying wood, soil, dung, or other decomposing material. - Such places have many microscopic lives (bacteria, tiny insects, etc.). - When mushrooms are picked and washed, many of these subtle beings can be harmed.

1. Anantkāy / many life‑forms together

- Like some “anantkāy” (multi‑life) vegetables, mushrooms are believed to carry or support numerous small organisms. - Eating them is seen as causing more collective himsā (violence) than ordinary above‑ground vegetables.

1. Difficult to clean properly

- Because they grow in damp and decaying places and have a spongy texture, it is hard to clean mushrooms completely. - Jains are careful not to swallow tiny insects/eggs or micro‑life along with the food.

1. Common practice in both sects

- Among both Digambar and Śvetāmbara Jains, most serious followers avoid mushrooms. - Exact strictness can differ by family or local tradition, but the general guidance is: “To follow Jain food properly, do not eat mushrooms or other fungi.”

So, the main reason is: To avoid harming countless small living beings and to follow ahimsā more strictly, Jains do not eat mushrooms.

You can read a short explanation here and also here