The core concept of The Invisible Man (2020) is based on the novel with the same name by H.G Wells. But he was ignored in the opening and closing credits. Why wasn't he credited?
Because it's not necessary
So there are no payments to be made to anyone nor is permission needed to use the title so there is no particular need to credit the author.
There are two reasons to include someone in the credits:
- Legal obligation
Since H.G. Wells's copyright has expired, there was no licensing contract between the studio and his estate for rights to make the movie--a contract that likely would have required the studio to credit Wells. Since titles alone (at least in the U.S.) are not subject to copyright, there's no issue there, either.
But the same could be said for Bram Stoker's Dracula (1992). Even at the time, Stoker's 1897 Dracula story was, I assumed, out of copyright in every jurisdiction. Yet he was not only credited; they put his name right in the title! This was marketing. An attempt to distinctly signal the potential audience that this is not yet-another-Dracula derivative, but an effort to portray the original story.
I haven't seen the new Invisible Man movie you've asked about, but my impression is that it's a bit of a departure from the Wells story, at least in tone, style, and time period. Prominently crediting H.G. Wells might mislead potential viewers who seek it out precisely because they want to see the H.G. Wells story and that they'd be disappointed, leading to bad reviews because of misplaced expectations. On the other hand, the title itself carries enough name recognition catch the attention of folks who might not have much of a predisposition and who might enjoy the present-day psychological thriller. Thus, from a marketing perspective, there existed a preference to not credit Wells, at least not prominently.
In addition to the legal reasons, let's consider two old movies with old source material
1931's Dracula was so successful, it spawned multiple Dracula/vampire stories not written by Stoker (and Bella Lugosi got typecast as a result). Sadly, it was a movie based on a play based on the book, which is part of why it eventually spawned a whole host of tropes about vampires, drinking blood, etc. And Lugosi's portrayal lives on in infamy as this scene from Hotel Transylvania parodies
In order to break out of the tropes, the 1992 film mentioned Stoker's name
The same thing happened to Frankenstein. The 1931 movie was also based on a play based on the book, which helped spawn a whole host of tropes. The movies eventually went so far afield from the source novel, the name "Frankenstein" is now believed to be the name of the monster (as in Abbot and Costello meet Frankenstein)
2020's Invisible Man could barely be called derivative of Wells
The Wells novel follows a man who somehow makes himself invisible and has various run-ins with the local village (vastly oversimplified for the sake of argument).
The 2020 movie is more like Hitchcock's Psycho than Wells' novel. It revolves around a woman who leaves an abusive relationship. The man she had been seeing leaves her money and apparently commits suicide. In reality he has become invisible and begins to stalk her (as well as gaslighting her). It is a psychological thriller where the title simply describes the main character of the movie.