A take from J. Dyson on why the Maxwell's EM theory took so much time to get traction.
Some notes that I learned from the HN thread is the importance of Oliver Heaviside on the foundation of the engineering tools we now use to deal in Electrical engineering.
One great quote in the thread:
Maxwell unified electromagnetism into a propagating wave. That was the big leap. Before that, things were disparate. Hertz proved it experimentally, and Heaviside turned it into an engineering discipline.