Short answer is, because loud and compressed music sounds better on playback systems consisting of cheap earbuds and tiny amps built into smartphones.
Longer answer...millennials are now the largest demographic in the U.S., and by far the biggest purchasers of music. Furthermore, file downloads and streaming together make up about 90% (my estimation) of all music purchases today. And how are the majority of millennials listening to music? On smartphones or portable players, with crappy earbuds and wimpy onboard amps. These things have very little capability as far as dynamic range, which means if you listen to something with a wide dynamic range on them, you're constantly fiddling with the volume control, as soft sounds disappear beneath the noise floor and loud sounds distort badly. Add to this the problem that music is usually consumed on the go, in noisy environments, with earbuds that do little to block it. Under these circumstances, music that is severely compressed and made louder just works better. Same goes for music in a car, though to a lesser extent because car stereos have better dynamic capability. Ever try to listen to a classical symphony while driving around town? You have to keep adjusting the volume control as it fluctuates from too soft to too loud.
For the rest of us who listen to music on proper playback systems, this sucks. You could wonder why they don't make CDs with normal dynamic range like they used to for people with home-based playback systems, while compressing and "loudening" the files and streamed music for iPhones. The answer is probably because it costs more money to make multiple masters, and streaming appears to be the future of music delivery.
Audiophiles like us are a dying breed, and a niche market. Very little music is being mixed and mastered for us anymore. Companies follow the profits, and apparently the major profits are in loud, compressed cloud-based music.