This is a very good questions. I remember asking it myself at once point. There are a lot of things that go into making abs.
This is the main key. If you are seeing leaness in your arms and your legs, you are on the right track. You either need more time, or your diet isn't good enough to get you lean to the point that your abs will show through. Abs are one of the last areas to go. Arms and legs are the first.
2. Ab work
Yes, you have to get lean enough to see your abs, but your abs also need to be developed enough to see. This takes time. Keep doing your high rep cable cruches and hanging leg raises, both for 4 sets of 20 reps, Do this at every other day. Maybe more often. I train abs for 10 min after almost every workout.
Understand, the guys on stage, or on the covers of magazines are not walking around like that year round. They may have abs, but the really hard/dry midsections that you see on the bodybuilding stage are the product of water manipulation, to look that dramatic for one specific day. Your abs can look greatwithout doing anything to water, but remember, the stage look isn't really what these guys look like every day
This is something you can't change. Everyone has a different shape to their abs. Some are like thick egg rolls, while others look like longer, flat, river stones. If you have longer, flatter looking abs, there is a chance they will never have that thick small look, but that doesn't mean that can't look awesome! Embrace your abs for what they are, and make them look their best!!