[bobdvb]

A significant part is that there's very poor data about how many people have surround sound systems or systems that can make use of such quality.

Sending it speculatively adds to the cost of delivery, but for a percentage of the audience it pushes their video quality down to the next resolution down. And for a percentage of the audience that'll be a more noticeable impact.

Here's another oddity: there's no great ways to measure audio quality subjectively. It's kind of been done for voice telecommunications but for perceptual codecs and media sound? The tools are terrible. So, quantifying decisions about how much bandwidth to allocate are hard. Most companies still depend on trained individuals ("golden ears") to test audio quality and for independent testing you need A/B testing with a listener panel. For video quality we have accepted tools to measure quality. They're not perfect but comparatively, any time you see an audio quality test tool you'll see a substantial professional audience that will happily dismiss it.

All increases in quality, audio or video, are subject to the law of diminishing returns. In audio the argument in favour of higher quality is far weaker than it is for something like HDR.