Is there something different in the muscle structure for males vs. females? Or does it have to do with hormones?
On a population level, women tend to have higher body fat percentage than men: 25% for normal women versus 15% for men. This means that for a man and women with equally-sized abs, the man's abs will look more prominent because they have less fat on top of them.
Another contributing factor is that men put on muscle more easily than women do. This isn't sexism, it's biological fact due to the actions of testosterone to build muscle. If a man and a woman exercise for the same amount of time and the same intensity, the man will build more muscle because he has about 3x as much testosterone as the woman, and androgens (including testosterone) increase muscle growth. (This is why some body builders take "steroids": to boost their muscle growth.)
So, in summary, the reason men develop "more abs" is because they have lower body fat and can build muscle more easily. This is of course ON AVERAGE...there are plenty of men who have no visible abs and plenty of women with impressive 6-packs. Any individual through proper diet and exercise can develop good-looking abs regardless of their gender.
First of All, women naturally have more body fat and it is healthier for a woman to have more body fat. We are naturally designed this way because we need the fat to help with pregnancies. We tend to have more belly fat for this purpose. It is a bit difficult for women to get 6-pack abs. Males and females need to follow different diets. Food affects men and women differently. Though men and women both need to do a different of styles of crunches, women need to particularly focus on diagonal crunches which target the lower abdominal section. It may not be that you are not working hard enough but that you are not eating the right foods and doing the right exercises. And also need to include cardio as well.
References: http://www.scientificamerican.com/article/why-does-fat-deposit-on-t/ https://www.sciencedaily.com/releases/2009/03/090302115755.htm http://slowyoga.com.au/its-ok-to-be-soft-why-womens-bodies-naturally-have-more-fat/