There could be so many reasons why:
1. Women are naturally "fixers." Some Christian women continue dating an unbeliever because she thinks she can change him. This is very common.
2. There seems to be more women in pursuit of relationship with God than men. There are Christian men out there, but go to a church on any given Sunday and you'll see mostly women. Because of this, Christian women are finding themselves in unequally yoked relationships. The wait to find "the one" can literally takes years, and this isn't an exaggeration.
3. Men aren't as vocal as women. There could be thousands of men in this type of situation but unlike women, they don't discuss their problems as openly.
4. Most men aren't as pressed to be in a relationship as women. A lot of women can't stand the wait and settle. The man ends up landing with Mrs. Right because he was able to wait for her.
I'm sure there are other reasons but these are the ones that stand out to me.