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.