I still think comparing romance novels to superheroes bears weight in that there’s little competition in the same fields as these three hold a near monopoly/duopoly over the market. There are or were superhero publications published by other publishers such as Image and Dark Horse but not too many of these survived in the long run, that’s without being bought by either DC or Marvel. This is likely true for romance.
Harlequin in fact bought some of its competitors (most notably Silhouette and Kimani). Recall how DC bought Charlton, Wildstorm (from Image) and that it was a merger between National Publications and Action Comics. For another matter, Marvel bought Malibu. There might be horror, crime, fantasy and science fiction publishers that did the same, just not to my knowledge.
That’s not to say there’s nary their equivalent of Harlequin or DC and Marvel. The closest would be Tor but there are several more doing the same or similar like Penguin, Gollancz and the like as far as I know. Maybe even the Oxford University Press for classics (romance included). If a publisher has a near-monopoly on the market, the market will be associated with it.
Same with a duopoly.