It was mostly because so many companies were built by Edison Electric and Thomas Edison was so famous many just kept the name.

From the start Edison was not planning to sell light bulbs, he was selling electric power utilities. It was a big business, in his name, but run a financed by a lot of people. Edison's companies sold power plants all over America and in Europe. By 1890 when Edison left the business they had sold 870 power stations.

Edison General Electric was merged with Thomson-Houston company in 1892 and Edison's name was dropped.... becoming General Electric. A lot of those old companies kept the Edison name because he was still the most famous guy out there.... still the Wizard of Menlow Park.