There is a podcast miniseries called The Divided Dial[0] that answers the question of why conservative radio dominates in America, but very briefly, based on my understanding of their reporting:

1. The elimination of the Fairness Doctrine meant that radio stations no longer had a legal obligation to provide a fair reflection of differing viewpoints on matters of public importance;

2. The elimination of national ownership caps in the 1996 Telecommunications Act enabled a rapid and extreme consolidation of radio stations;

3. These new national radio conglomerates slashed costs by vertically integrating production, creating fewer shows, and rebroadcasting them to all their owned stations;

4. The concept of “format purity” spilled over from music radio into talk radio, causing commercial talk stations to switch from showcasing a variety of opinions to airing one political perspective all day;

5. The conservative talk radio format was perceived as less risky by radio executives, and so that was the format that commercial talk radio switched to.

Air America may have eventually succeeded despite its many other flaws—except they owned no radio stations of their own, so there was no place for them to go in this hyper-consolidated, format-pure commercial market.

[0] https://www.wnycstudios.org/podcasts/otm/divided-dial