> "Commercial" doesn't really mean anything in this context, as presumably everything that you listen to on the radio/streaming services is commercial.
It means dumbed down to the lowest common denominator. It means being told that instead of deeply personal, complex, and potentially offensive lyrics you're just going to repeat a word or small phrase over and over and over because that sells better doesn't alienate anyone with difficult words or things that might offended them, or might discourage the use (and sale) of that song in commercials, grocery stores, overseas markets, or film soundtracks.
Commercial interests (the desire for as much money as possible at the expense of all else) overrides what would have been deliberate artistic choices in order to maximize profits leading to a landscape of homogenized bland overproduced and unchallenging art.
We've seen many changes in music as a direct result of commercial interests overriding the preferences of artists (and listeners) including the loudness wars, the end of the album as a cohesive work to favor itunes downloads of single tracks, and the lengths of songs getting shorter and shorter to the point where bridges and entire verses are removed.
Not everything you hear on the radio has been fully compromised by commercial interests, but it's a good bet compromises were made and commercial interests have had more influence over what gets heard on the radio than any other factor (talent, skill, popularity, artistry, etc.)