Why do historians organize history thematically?

Answer 1

Laziness. It's easier to specialise in the history of napkins in 19th-century Charleroi than to study the many factors and processes at work over vast areas and periods and come up with a coherent in-depth analysis of the broader sweep of history. OK, that's unkind, as the mountain of evidence, analysis and interpretation available to today's historian routinely exceeds the ability to digest and assess it, and as the skills needed become ever more technical: for that matter over-general "mega-history" can be a dire affair. But thematic fragmentation threatens the cohesion of history as a discipline: one leading economic historian has complained of the field's takeover by economists at the expense of historians. It's a trend that appears likely to continue.