This is a good question. First, you should note that the major expression of Romanticism in the English novel was called the "Gothic" novel, and thus when we talk about Romantic qualities in the English novel, we often look at Gothic elements, such as medievalism, sentimental affinity to nature, intense portrayal of emotion, exotic landscapes, etc. Walter Scott is probably the most important Romantic English novelist.
There are several ways to approach this. First, literary history is not linear. In the case of some technology, progress is linear. Few of us would want to use a 25-year-old computer if we could avoid it. On the other hand, we can still enjoy reading books that are hundreds or even thousands of years old. Just as in popular culture there are many forms of "retro" movements such as Steampunk and the Society for Creative Anachronism, so too writers can draw inspiration from older sources and imitate older styles they feel drawn to.
The Gothic itself, after all, was named after a term used to describe the Middle Ages, and some of the original Gothic novels were set in that period. Walter Scott, for example, drew extensively on earlier periods of Scottish history. Charlotte Brontë's Jane Eyre marks a transition between the older Gothic style and the newer sensation novel. Although it borrows many distinct Gothic features, such as the use of the supernatural (usually explained away), the poor but virtuous girl who turns out to be an heiress, the dark mysterious stranger with a secret (Rochester), and the strange deserted mansion, it is still more domestic than the true Gothic, set in England, and focusing on the bourgeoisie rather than the aristocracy. Brontë's use of these elements is part of her own interest in very strong, melodramatic emotions; she herself seems to have a Gothic sensibility rather than the witty and rational temperament of Jane Austen or the genial but perceptive nature of someone like Trollope.
Eventually, individual authors follow patterns which appeal to their own artistic nature. Just as some people like the latest trends in music and some prefer music of other periods, so too some authors like the major trends of their periods and others rebel against them.