When Frankenstein was first published in 1818, it had mixed reviews. Not everyone understood or enjoyed it. Today, however, we think of the book as a classic. Speculate about why some important pieces of literature are not treasured right away, while others that are immediately popular eventually fade away.
When books are published and released to the reading audience, they enter into an existing historical moment and culture, both of which have a set of discourses that characterize and inform what people believe. When Shelley published Frankenstein, the age of industrialization was upon Europe, and popular discourse held that machinery would make human life better. However, Shelley implicitly criticizes man's attempt to create and suggests some of the dangers in doing so. That is one reason why some did not appreciate her novel--it went against the popular discourse of the time. Now, however, as contemporary readers question the role that man should play in harnessing nature, a novel like Frankenstein is more appreciated because it grapples with questions that we are considering in our lives.
The same holds true for novels that are immediately popular, but fade away--they capture questions and issues that are of immediate concern, but as those issues fade, so do the novels that discuss them.