16:9 is a more "natural" ratio; ie. it comes closer to how we see the world. This is useful if you wish to have an experience that fills your field of view as much as possible, like watching video or playing games.
However, while this fits our field of vision for just looking at stuff, it is totally different from our optimal field of vision when reading. Text can very quickly become too wide to read comfortably (or write), and having more text displayed vertically helps when scanning through a big body of text.
If we were only talking computers, you could argue those two usage patterns would drive the market in two directions. But when you add televisions, which are not used for working on, the market gets driven squarely in one single direction. There will be holdouts because the market for narrower, taller screens remains in existence, but it is the smaller one of the two.