"Perhaps the monk, Heathcliffe, and the Beetle are so interwoven into their historical novel context that they cannot be released from the pages. In contrast, Frankenstein’s monster and Dracula have been successfully uprooted..."

Perhaps. I'd suggest there's a specific additional factor: Frankenstein and Dracula were both adapted for theatre, each text receiving numerous stage interpretations. Many more people knew Frankenstein's story as a stage play than as a novel in Shelley's own time. (William St Clair's 2004 book The Reading Nation in the Romantic Period has a whole chapter on how Frankenstein's adaptations and copyright status shaped its reception history and current popularity.) And the earliest Dracula adaptations were often paired with Frankenstein adaptations, and subsequently these characters were cast in plays and films together (Universal Studies had great successes with its Frankenstein and Dracula film franchises that culminated in "monster mash" movies featuring multiple iconic creatures).

Frankenstein was shocking to readers, but its stage adaptations significantly simplified the story into a Faustian pact-with-the-devil kind of morality play (an angle that Universal kept up). The Monk also received stage treatments; but David Christopher suggests a few reasons why The Monk wasn't as adaptable to a theatre culture ruled by melodrama:

On 29 December 1798, John Philip Kemble staged James Boaden’s Aurelio and Miranda at Drury Lane. The play was an adaptation of Matthew Gregory Lewis’s novel The Monk (1797) which involves a famously pious monk who is tortured and damned after being seduced into committing the most heinous crimes. The novel also includes a substantial subplot involving star-crossed young lovers and the ghost of a Bleeding Nun. Various aspects of Lewis’s text constrained Boaden’s adaptation. Any attempt at an adaptation of The Monk was already problematized by the public opinion of the novel as notoriously irreverent, and the unapologetically graphic physical and psychological horror it contained.The intense psychological aspect of Lewis’s novel did not marry well with Boaden’s methodology, nor did Lewis’s plot meet melodrama’s generic needs of romance and a happy ending. Boaden was left with the overwhelming task of creating a plotline that satisfied these needs from a less-than-ideal source, but that superficially appeared to satisfy his propensity for its Gothic content. While both prose fiction and drama made use of Gothic conventions, the drama was encumbered with a socially supported censorship that maintained absolute standards of eighteenth-century morality, and an uncompromising melodramatic formula which required unambiguously evil villains, the punishment of vice, rewarded lovers, and a unified plotline.

Similarly, The Beetle was adapted to stage and screen (here's a brief summary of those adaptations). Yet while Marsh's novel outsold Stoker's -- both were published in the same year -- Stoker's would enjoy a much richer tradition of popular adaptations, and a much wider readership, in the long run.

It's a foundational principle of the popular culture market that nobody really knows what will sell until it's out there. (Sequels are Hollywood's way of making educated guesses about what will sell.) Something about Frankenstein and Dracula resonated sufficiently with readers and theatregoers to propel their popularity over and above that of The Monk or The Beetle. But producers' bet-hedging and sales records also play a part: Frankenstein's theatrical popularity had been established for most of a century before Dracula emerged and grabbed Frankenstein's coat-tails. Neither Lewis' work nor Marsh's got connected with Frankenstein's successful adaptation tradition the way Dracula did, which may suggest why they haven't achieved the iconic status of Frankenberry or Count Chocula.

Thanks for the thought-provoking response. That brings books such as Twilight, Harry Potter, and Series of Unfortunate Events and their subsequent adaptations under a similar lens. Theatre and screen adaptations influence today's books and their success in popular culture as well. How much? Further, I wonder if there is some solid formula that can be followed for success. Pairing two monsters brought success for Universal Studios, as is pitting Marvel's greats, Captain America and Ironman, against each other in the Avengers. Perhaps Moneyball's premise has merit: maybe there is some kind of specific formula in some arenas that can be followed to reach great success. In the publishing and entertainment world, perhaps such a formula is followed more often than we realize. 

Great post -- I especially like how you observe similarities between the suspicion of novels in AUsten's time and the suspicion of new media forms and texts today. Current critiques (or jeremiads) against digital communication platforms and genres posit shallowness or worthlessness in the name of older forms that are purportedly deeper or worthier; however, there are some fascinating counter-critiques of such positions -- for instance, the Stanford Study of Writing, which makes some interesting arguments for the vaslue of new media forms and platforms in the teaching, practice, and promotion of writing.