Why did Buddhism despite being founded on shared doctrines and principles fragment into multiple schools so early in its history?

Early Buddhist history shows a rapid diversification of viewpoints, lineages, and monastic communities, eventually producing schools such as Theravāda, Sarvāstivāda, Mahāsāṃghika, and many others. What historical, doctrinal, social, or organizational factors contributed to this early schism? To what extent were these divisions driven by philosophical disagreements, regional spread, monastic discipline (Vinaya) issues, or broader political and cultural influences in ancient India?