Short answer: because of ahimsa (non-violence).

• In Jainism, all living beings, even tiny microorganisms, are deserving of non-harm. Mushrooms are fungi, not plants, and their growth and harvesting can involve harming microscopic life in the soil or substrate.
• Many Jains therefore avoid eating mushrooms to minimize harm to life forms that may be present in or around them.
• Practices vary by group and family. Some Jains, especially among stricter households, avoid mushrooms, while others may eat them if they are confident about minimizing harm. There isn’t a universal ban across all Jains, but the emphasis on non-violence leads many to refrain.