Regarding her choice of nom de plume, Charlotte Bronte herself wrote,
Averse to public publicity, we veiled our own names to Currer, Ellis, and Acton Bell.
In addition to realizing that it would be difficult to get their work published if they put their real names, Charlotte and her sisters felt that their writing at the time was not what would be called "feminine" and they would be "looked on with prejudice."
Charlotte Bronte was writing her works at a time (the 1840s) when women authors were not taken as seriously as male authors. Bronte started to use a male pseudonym when she published her and her sisters poems under the names of Currer, Ellis, and Acton Bell, male pen-names. Bronte pretended to be a male and used male narrators in order to project masculinity, which would broaden her audience (because some people might not want to read a book by a woman or primarily about a woman), be taken seriously, and have people respect her work more. In some ways, projecting that male-ness on a piece gave Bronte more power, power of influence, and power to explore gender issues more thoroughly.
One must understand the era in which Charlotte Bronte was writing to see why she would use a male pen name. The 1840s was a time in history when women authors were not perceived to have the credibility that male authors had. Women were thought to be frivolous, frilly, and dainty creatures who had no perception of "the real world." This was a time when women's roles were that of mother, wife, housemaid, and little else.
It was not uncommon for women authors of this period (and others) to utilize male names to enhance their reception by editors, publishers, and others in the male-dominated industry. In contrast, today's literary marketplace is largely open to authors of every gender and persuasion.
Is it possible perhaps that Charlotte Bronte wanted to hide herself behind a male persona as she actively wanted to shun the limelight? She had great respect for those women who could talk in a forthright manner in public and not veer from their beliefs in the face of public opprobrium, but she was not one of these women. Her depressions and hypochondriasis would lead one to believe that she was a highly strung woman aware of her own emotional limitations, who dreaded being unmasked and open to criticism aimed at her personally not her creation Currer Bell.