The mind

Why do some excel at martial arts while others do not?

Certainly your physical abilities play a role in your progression, but it's your mind that drives the body.
Therefor your mind, the way it works and how to get the most out of it dissevers particular attention.

How decisions are made.
Every action you do or think of is processed by the brain in a democratic manner. The brain considers the price and benefit of applying the action, the final movements quality is dependent on the votes outcome.
Lets say an Instructor tells a student to throw an attack at him. The student will throw the punch considering the following.
1. If I don't throw the punch my teacher will be upset or angry.
2. If I throw the punch I might get hurt by the counter.
So the student now has a certain number of neurons firing in the brain to throw the punch but also a number trying to stop the action. All dependent on the price if he does or doesn't and the benefit if he does or doesn't.

The price if he doesn't may be the humiliation of the teacher and class members towards him. This is weighed against the price if he does throw the attack and the pain he may receive from the counter.
The benefit of not throwing the punch would be not risking injury at that moment against the benefit of throwing the attack and maybe earning admiration by fellow students or the instructor.
Certainly many other influences could be considered, but the general idea is there. Price and benefit are the major factors to the quality of movement, due to the conflict it creates within the brain.

Focus.
The amount of price and benefit one considers towards an action dictates the amount of focus used for that action. If you were sparring with a partner and were sure that your initial attack would be countered, the benefit may not be high and therefor you would not place all your efforts in applying it. In other words you would focus more on a second attack. Compare this with if you were sure that you could land your first punch, the benefit now is high and you would surely give more focus and effort to the initial attack.

Have a look at the image below.

You will see a Duck or a Rabbit. Notice when you focus on one the other disappears. This is an important fact of Focus.
With focus you filter out other material.

Here is another example.

It's possible to see three options above. Can you make them out? As you see one you cannot see the others. Only one is possible to focus on at one time.

1. A box in the corner of a room.
2. A solid larger object with a cubical hole or section missing.
3. Two solid objects with the smaller one in front of the larger.

Lets take this into practice.
If you were sparring with a partner who you considered higher skilled than yourself, you might be focusing on your defences or on your partners attacks. Remember you already made the decision that your partner would probably win the round as your skill level is not as good.
While your focus is on loosing, or defending, it cannot be on winning or attacking.
But its still possible to win in this situation as your partner also is bound by the filtering rule. Distractions are the main proof of the filtering rule, if you can cause your partner to brake their focus then they would be vulnerable. By doing something unexpected the opponent would have to focus on it and decide whether it is a threat or not and then apply a strategy against it. Their focus has now shifted to defence and for a moment you have shifted yours towards attacking and winning.
Certainly if the opponent is extremely talented, the distraction may not work as they have experienced it before, but just think for a moment. Have you ever seen or heard of a fighter loosing to a more junior fighter. It happens. Why? Because the better fighter was not properly focused and the junior was.

If you decide that you will loose, then you will loose. That is where you are focused.
What you focus on, you excel at.

Take that a little further. If you asked yourself "why do I always loose to this partner?" your brain will give you many answers because your focus is on finding reasons why you are not as good as your partner. A better question to ask of your self is "how can I confuse, trap or strike my opponent?" Now your focus is much more practical, as the mind is focused on solutions.

Your focus is governed by your rules.
We all have certain rules we live by.
As an example, as a Wing Chun student you might believe that high kicks are ineffective or dangerous to apply, due to the close proximity to your opponent and efficiency, so you focus on your hands much more than kicking high. Now some other schools train a lot of high kicks, do they have the same rules? There is no right or wrong here, remember the images above, once you focus on one you tend to filter out the other.
We need to be aware of some of the rules we have set for our selves. Sometimes the rules are set unintentionally. You may have had a bad experience applying a particular technique one class, say you got hurt as a result, you might associate the pain with with that technique especially if it happens again . Imagine how the brain will vote the next time you consider using the technique. Technique = pain, so even though you focus on the technique your internal rules govern how you will apply it.
As an experiment think about a technique you do not use a lot and try to define the reasons why, not just because it doesn't work, dig deep to find any association you may have created.