It's the same reason why I've been saying that Gaga is over. She's been everywhere the last six months or so, and "Shallow" hit #1 on the Hot 100. But guess what? Young people don't care. Her streaming numbers were almost non-existent, and old channels (iTunes, album sales) are what have fueled her success the last several months. She's not reaching the young people, and that's why A Star is Born is her going-away party.
Obviously Beyonce and Taylor Swift aren't at that point yet (....yet) but a sure sign that you aren't reaching young people is to look at your streaming numbers. That's almost entirely where young people consume their music - not radio, not iTunes, not the CD sampler at Virgin Records. The paradigm has shifted, and artists from a different generation have got to get on the trolley.
Taylor's debut single from reputation did well for a bit, but then what? It's like that album disappeared.
If you have all the young people not listening to your music because you've neglected their platforms, your career will be fucked. Because every year that passes by, that audience becomes the tastemakers in our culture.
Taylor and Beyonce are already selling out stadiums - it will cost them nothing to shift more extensively toward streaming.