First of all, with a low female population there probably aren't enough proven and qualified women to fill the desired number of slots. This leads to imposter syndrome / worry about people thinking "she only has this position because of gender". It also leads to these women sometimes doing poorer jobs, just because there aren't enough qualified women in the company to fill those positions.
Secondly, you drain the lower rungs of the company from women. This leads to under-representation in the lower rungs. Meaning fewer women getting the training time one needs to properly move up the ladder. It also means a dearth of female perspective on the lower rungs. Hurting both diversity of views, and the culture. That second point makes the company less attractive to women trying to enter the company, which perpetuates the problem.