Why did Einstein feel that Newtonian mechanics were too simple for electromagnetic fields?

Newtons' theories had been the standard, the accepted way of understanding motion with his description through example equations, of the physical properties of the world for some two hundred years before Einstein came on the scene. Then firstly, Newton's theory of motion was undermined by Maxwell's discovery of electro-magnetism in the 19th century.

Then Einstein's theory of relativity further undermined Newton's theory of gravity. By 1905 he worked out that electro-magnetismthe combined with the later Lorentz transformation meant that time and space are no longer absolute/ They change their properties when changing from one inertial frame to another.