Because it's a crappy version of the count of monte cristo.
They both have connections to the criminal underground, they both anger the husband of their love causing death.
Not exact duplicates by any means, but the similarities are there.
Besides that, there's more to those books than the main plot. I always assumed that TGG is this well regarded because of themes it handles and how they're presented by the story of Gatsby. The same goes for The Count of Monte Cristo. I find them to be too different to call on even a version of the other, let alone a crappy one.