Several of the companies erred by getting one thing wrong: they overpaid with borrowed monies in the pre-Internet and pre-smartphone expectation of continued revenue growth.
When growth stalled in the early 2000's, and then the recession hit in 2008 (coupled with changes in ratings measurements in the highest revenue markets) the premise failed, and the companies with excessive debt had to take extreme measures.
However, the two major failures were due to other factors: Clear Channel was bought at the height of LBO acquisitions by investment banking interests at way to high a price, and Cumulus was the victim of a mismanaged amalgamation of a large market group with a fundamentally smaller market company.
While there have been other bankruptcies or distress sales, that is nothing new and instances of that in radio go back to the 50's at least.
On the other hand, more prudent operators did well. Bonneville, Hubbard, Saga, and Beasley are examples. and there are smaller regional groups that are prospering thanks to being able to have clusters of stations in a variety of medium and small markets.
In fact, the larger sell-offs have been done not due to the lack of profitability but the desire to get out of radio due to wall street pressures for accelerated growth, something radio does not offer (and the transformation into new media enterprises is hampered by the lack of a profitable business model in that field).
Examples: ABC did not grow in the consolidation years of 1996 to 2000; they found themselves in a weak competitive position in very large markets. Lincoln Financial sold because the parent company underwent a merger and radio was spun off to finance, in some part, those changes. Susquehanna was sold because the heirs to the Pfaltzgraff fortune wanted to "see the money". Greater Media was sold, also, due to a later-generation of the family not wanting to run a radio company.
Remember that consolidation came late in the US. In many parts of the world, ownership of multiple stations in the same market has been possible since the 50's and 60's, and was, for half a century a formula for success.
So, really, radio's problems and issues are not caused by bad management of owners big and small but by changes in the advertising markets caused by the growth of digital, a competitor nobody could envision in 1996.