One excellent account and explanation of these developments is Geoff Eley’s A Crooked Line. Part historiography, part biography, part political history, this book follows Eley from 1960s Oxford to Germany to the US, spanning the origins of social history to the riposte of cultural history to Eley’s predictions — ultimately borne out I believe — of a rapprochement between these approaches. Anyway, the first chapters help to answer your question. Elegy points to such contributing factors as Marxist political leanings emerging among young British historians in particular, the influence of the French Annales historians, and overlaps between anthropology and other social sciences and history. But he especially points to changes in WHO could be a historian. Elegy focuses on women historians as particular pioneers, who as they broke into the profession began to ask different questions that then changed HOW history had to be done to answer them. Historians of minoritized communities also. I would add the emergence of professional historians from the “colonies” just reaching independence. These new groups, I would argue, pushed the field to change. Not least, they found that in order to study the groups they represented, the “from below” methods were necessary since these groups were largely written out of the archive and the corpus, or at least marginalized within it.