In past comments I've written about the symptoms and observed effects of wealth concentration, but I don't think wealth concentration explains this. It's a bad fit.
Instead, I believe abundance of supply does. In the past, there were simply fewer things to subscribe to, so a median family would pay for the local newspaper and cable -- and generations before then, just the newspaper. Rich professionals and the elite would pay for one of the premier papers, and finance paper, and an arts magazine. Fans with particular interests would pay for one or two other serials.
These days, content is everywhere, and much of it is available for free. This is impossible to compete with on price, and has been shown to be hard to compete with on quality and depth of coverage, because it chops off large portions of the market who may have paid for convenience of distribution in the past, but now they don't have to. This leaves even more pressure on those willing to pay. In this kind of situation, the classic resolution is consolidation, which is already underway.