Umami was named/Identified in Japan in 1908. That's still a pretty recent discovery compared to sweet and salt and the human timescale of discovering and naming

Umami was named/Identified in Japan in 1908. That's still a pretty recent discovery compared to sweet and salt and the human timescale of discovering and naming flavors. So it isn't surprising that it is still working its way into people's consciousness and vocabulary. It is also subtle compared to the big 4 tastes.