I’ll answer in a simple, Jain‑Dharma–faithful way.

First, from the Jain point of view:

• Jain Dharma itself never decreases or ends.

It is an eternal (anādi‑ananta) truth. Only its visibility in society and number of followers go up and down with time.

When people say “Jainism decreased”, they usually mean:

• There are fewer Jains in many regions than before,
• There is less political power and public influence than in some past periods.

Historically, some main reasons:

1. Change in royal support (rājāśraya):

In ancient and medieval India many kings strongly supported Jain Dharma, built temples, and protected monks. Later, many rulers started supporting other paths (various forms of Hindu bhakti, later Islam etc.). When kings changed their support, Jain temples, pathshalas, and sanghs in those regions became weaker.

1. Violence, invasions, and temple destruction:

In some periods, especially in parts of North and South India, wars and religious conflicts led to: - destruction or damage of many Jain temples and basadis - loss of old Jain cities and trade centers - disruption of monastic sangh routes and support This reduced Jain presence in those areas over time.

1. Very high standards of practice:

Jain Dharma has very strict rules of ahimsa, aparigraha, brahmacharya, etc. For many ordinary people, such high discipline felt too difficult, so they shifted to easier paths. Result: Dharma stayed pure, but the number of followers grew slowly or decreased in some areas.

1. Internal divisions (Digambar–Shvetāmbar and sub‑sects):

After Bhagwan Mahavira, differences in following certain rules led to: - separate traditions (mainly Digambar and Shvetambar) - later, many sub‑sects inside each This did not destroy Jainism, but it did weaken united social strength and outside influence.

1. Rise of other traditions around Jains:

Over time, powerful bhakti movements and other sampradāyas grew. Some Jain ideas (like ahimsa, karma theory, tapas) were absorbed into other Indian paths, so many people accepted the values but not the formal Jain identity.

1. Modern lifestyle and materialism:

In recent centuries, even many born Jains: - became busy in business/profession, - weakened in daily sādhanā (pratikraman, swādhyāy, upvāsa, dāna), - adopted more worldly habits. So the depth of practice decreased, even if names and temples remained.

Very important Jain view:

• Jain Dharma has not “failed” or ended.
• Wherever one soul truly follows samyak‑darshan, samyak‑gyān, and samyak‑chāritra, there Jain Dharma is fully alive.
• What we call “decline” is mainly:

- loss of political power, - loss of numbers in some regions, - and decline of actual practice among many who still call themselves Jain.

If you wish to read more in simple language, you can see this explanation on JainKnowledge: “History of fall of Jainism”