Very simple: high availability of music production equipment. As equipment gets cheaper and easier to use (e.g. music production software), it naturally becomes more ubiquitous. Subjectivity aside, the bulk of music is derivative and unexceptional. It many ways, it could similarly be argued that music has gotten better (it terms of production and overall composition), based on the above alone. A case in point: electronic music (even beat tapes) used to be very labour intensive until even as recently as 2005-2010. This is no so now. In my opinion, popular music has and always will be derivative and drivel (with many exceptions). Of course, we notice it more as popular music actually becomes more popular and reaches wider audiences. Big artists can drop a track on YouTube and hit hundreds of millions of views in weeks or even days. Compare that to needing to watch a weekly programme on TV or tune in to the radio just to hear what other people are listening to only a couple decades ago. By the same token, good music is easier to produce and release. Definitely for me, the majority of my favourite songs are from the 21st C, even though I like classical and blues.
