Linear motors were invented in 1895 but practical machinery was not developed till about 1947. The British Engineer Eric Laithwaite did most of the research between 1950 to 1960.
The problem with Linear motors was the cost, the amount of energy required and the difficulty of the development of the large electromagnets. This hampered the development.
However, with the advent of "Superconductivity" things changed. If electromagnets are cooled to low temperatures using liquid helium or nitrogen their electrical resistance disappears almost entirely, which reduces power consumption considerably.
So all those trains that use MagLev are based on superconducting magnets.
The leaders in this area are the Japanese & the Germans.