Why did most American record companies stop making monaural recordings by the late 1960s?

By the late 1960s, most American record companies had discontinued monaural recordings—with the exception of historical reissues—primarily due to the widespread adoption and commercial success of stereophonic sound systems.1

Following the introduction of the stereophonic disc in 1958, the industry saw a surge in demand for recordings that could showcase the spectacular directional and acoustic effects made possible by stereo technology. As music recording techniques evolved, producers increasingly utilized multi-track recording and sophisticated mixing to create immersive soundscapes, moving beyond the limitations of single-channel audio. Because stereo offered a more realistic sense of concert-hall acoustics and instrument placement, it became the preferred standard for both consumers and labels, rendering monaural production largely obsolete for new releases by the end of the decade.123