Why did the theremin, ondes martenot, and trautonium become mostly obsolete?

The theremin, ondes martenot, and trautonium became largely obsolete because the specific sounds they produced could be easily replicated by electronic music synthesizers.1

While these instruments were pioneering inventions of the early 20th century that provided novel timbres for avant-garde composers, they were monophonic, meaning they could only play one melodic line at a time. As technology advanced, the development of synthesizers—which offered greater versatility, polyphonic capabilities, and more convenient interfaces—rendered these earlier, specialized instruments less practical for general musical use.12

It is important to recognize that while these instruments fell out of common use, they remain historically significant for their role in the evolution of electronic music and for the specific compositions written for them by notable figures such as Olivier Messiaen and Paul Hindemith.31