Because female orcs existed. Tolkien even confirmed it in 1963 in a letter to a woman named Mrs. Munby.

http://tolkiengateway.net/wiki/Letter_to_Mrs_Munby

Here is where it is confirmed in the letter.

The letter is long, but in one place reads as follows: 'There must have been orc-women. But in stories that seldom if ever see the Orcs except as soldiers of armies in the service of the evil lords we naturally would not learn much about their lives. Not much was known'. Tolkien also goes on to discuss the use of the word 'goblin': 'In The Hobbit 'goblin' is used... but goblin is a fairly modern word, and very vague in its application to any sort of bogey in the dark.'