What key 19th-century developments necessitated the move toward standardized time zones beyond local solar time?

Management of railways and telegraphs

For most of history, time was measured as local solar time, meaning noon was determined independently by each community based on the sun's highest point. This system was feasible when travel and communication were slow. However, the arrival of reliable, highly accurate mechanical timekeeping—especially the chronometers like those developed by Harrison—and the subsequent industrial expansion made local variance unmanageable. The necessity for standardized time zones arose primarily in the late 19th century to coordinate the schedules and logistics of rapidly expanding transportation networks, specifically the railways, and the near-instantaneous communication provided by the telegraph system. Mechanical clocks began to enforce temporal uniformity over distances.