In Jain Dharma, food is chosen by the principle of ahimsa — avoiding harm to the maximum possible extent.

So the difference is not simply “has seeds or not,” but how much violence is involved in eating, cutting, and preparing it.

• Cucumber is generally accepted in Jain diet because it is considered a simple fruit/vegetable with comparatively less harm.
• Jackfruit is often avoided in many Jain traditions because it is large, sticky, difficult to separate cleanly, and contains many seeds and fibers, so it is seen as involving more harm and more disturbance to life.

Also, Jain dietary rules can vary by sect and local tradition. Some communities are stricter than others.

So the main reason is: Jains avoid foods that are believed to cause greater harm, not just foods with seeds.

If you want, I can also explain this in simple Gujarati or Hindi.