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The simple answer to why is to increase the labour supply to decrease wages. Programming jobs are some of the highest paid jobs that employees can find right now, and people are looking to correct that. This is the heart of the matter.

The sightly more complex answer to why women (or girls) are singled out specifically is that in a male dominated field we can assume that the best males, who have any interest in working in said field, are already working in the industry. Attracting more males simply reduces the quality of the supply. When women are a virtually untapped market, you can attempt to bring in the best women, thus increasing the numbers without affecting the quality of the supply.