"What is likely to have happened is that the Buddha changed the way he taught. This would be quite appropriate given the rapid change and development of his following over the years. In the early times there was a small group of dedicated, attained followers, while in later years you had many less dedicated, less intelligent followers. In addition the seniors had already learnt the basics thoroughly and wanted more detailed teachings (e.g. the Mahanidana Sutta); and there was increasing specialisation in different areas like Vinaya, systematic analysis (proto-Abhidhamma), or lay teaching. Sujato's Blog