As someone who is a 'high-achiever', I would actually argue that motivation and effort only covers some of the reasons why people don't do well. I have personally seen a lot of people, bright people, who are undermined by their mental health. This is around the same age that mental health problems begin to develop in most people who will have them, and it can have a significant impact. As someone who has had depression and recovered, I can tell you that during the period where I had depression, I had a medical note to excuse me from homework because it was too much, let alone revision.
On the other hand, though i typically get A* and A's in most subjects, I get U's in French. It doesn't matter how much work I do, I always end up with the same poor marks. Obviously, as this is such a difficultly for me I don't focus on it for actual GCSE's, but I was aiming for a C, and even then i feel short. Whilst at the same time I'm predicted A* for everything else and have even achieved it in exams I've already done.
Also, some people don't do well with exams. The brightest person I know, who always knows the answers to everything and will research things due to a genuine joy of learning and spends hour or revising can't manage to get above a B in most exams, even the ones they naturally excel at. This is the case for a lot of people.
Alternatively, people get too stressed out and they just shut down. GCSE's might be 'easy', but they do require a large work load (especially if you do art) and this can drain people. It also puts limits on time that can be spent revising. Not to mention you have teachers telling people these are the most important exams of their lives etc. Also, my school is still giving people out homework and we're not allowed to go on study leave, which is negatively effecting everyone.
Furthermore, some people don't know how to revise. Everyone revises differently, but you never really get taught how to do that, especially if you have never needed to revise before. This means you could do hours of revision but end up knowing no more at the end of it. This has happened to a lot of people I know. Some people can't even function in a class room environment because it doesn't benefit their learning style, and they then have understood nothing in class so don't even have a basis.
Also, it is all well and good to say you don't understand how people can't achieve something, when you have an ability at something. I myself have looked around and wondered how people don't understand what seems like the simpliest thing to me. You have to realize not everyone experiences the same thing as you, or has the same mental capacity. It would be like a great artist wondering why no one else can paint a masterpiece.
Finally, I do think that teachers play a role. If you can't get something in class, it often discourages people. I'm lucky in the fact I often do well in an academic environment, but when i do mess up or don't get something, I end up getting really wound up and annoyed and frustrated. For people who struggle to understand a lot of things, this would be very emotionally exhausting. Also, teachers can put people down. My mum quit school because the teacher said she was never going to achieve anything, so my mum couldn't see the point. I myself was actually called an utter disappointment the other day by a teacher (which is always fun.), and when I was younger I was put in the lower sets and labelled as stupid due to a speech impediment. I was only ever considered bright after the year 2 sats, which forced them to move me up. I was too young to particularly care, but I think it means I really value my academic ability. So it can go in two ways.