Here's why:
* Shelley's reputation: Percy Shelley was a well-known poet and writer, already established in literary circles.
* Mary's youth and gender: Mary Shelley was only 19 years old when the novel was published and women were still considered less capable than men in intellectual pursuits.
* The novel's themes: The themes of the novel, particularly the critique of scientific ambition, resonated with Shelley's own philosophical and political beliefs.
Although Mary Shelley was the true author, she initially chose to remain anonymous. It wasn't until later, with subsequent editions, that her name was officially credited on the title page.