Why was the phrase It don't mean a thing if it ain't got that swing popular?

It was popular because Duke Ellington, a renowned Jazz artist, was able to capture the essence of rhythm in that phrase. It was a way of saying, your music is nothing if it can't involve the listeners. Now we have contemporary music that sometimes detracts from his definition, but the key point is that everyone wanted to have that "swing" to get their songs up to the top. Saying that phrase meant you knew some about music and that you appreciated a song that could capture your attention and get you moving. Criticism of music is important in elevating it to better, more complex levels. When Duke Ellington said, "It don't mean a thing, if it ain't got that swing", it stroke a chord, if you will, with both listeners and musicians to be critical with music in order to undertake more difficult and complex production of songs and support those who can produce such music.