I wonder whether they didn't sell well. The covers seem more attractive to male readers -- who probably weren't buying romance books -- than to female readers -- who were the main buyers of the books.
I agree.
Check out these covers from 1952 - 1954 to the venerable Love Story Magazine, the long running pulp dating back to 1921. We are well before the "Fabio" era where hunk-age is a de rigueur element on Romance covers. At this stage, the industry still helped the reader project herself onto the main heroine on the cover.
Maybe, it simply wasn't wholesome enough and there is too much of a come-and-see me style to the cover combined with the Forbidden that was just enough askew that the main clientele simply didn't want to be associated in any way, shape or form with the package. Knowing that the male client weren't interested in the first place, the title died a quiet death on the newsstands.
These are beautiful covers!
Oh dear... please don't get me addicted to another genre...