Why did the Brontë sisters publish as men?

Because they expected to be judged as women rather than as writers. Charlotte Brontë explained it herself: the sisters were "averse to personal publicity", and although they did not think their writing was especially "feminine", they "had a vague impression that authoresses are liable to be looked on with prejudice". So Charlotte, Emily and Anne became Currer, Ellis and Acton Bell, names that kept their own initials.

The fear was not imaginary. At twenty, Charlotte had written to the Poet Laureate Robert Southey about her dream to "be forever known". He replied that "literature cannot be the business of a woman's life, and it ought not to be". She thanked him and kept writing anyway. Even as children, girls were not allowed in the village library, so their brother Branwell shared his books with them.

The disguise worked, at first. Their book of poems sold only two copies, but in 1847 Jane Eyre by Currer Bell was an immediate success. Then Wuthering Heights and Agnes Grey appeared, rumours swirled about who the Bells were, and critics' tone changed: they began calling the writing "coarse", a shift linked to the growing suspicion that Currer Bell was a woman. In other words, the critics proved the sisters right.

In July 1848, Charlotte and Anne took the train to London to prove they were separate authors. Their astonished publisher found two frightened country women holding letters addressed to the Bells. Tragically, Emily died that December and Anne the following May, both of tuberculosis, before their novels were recognized as masterpieces.