Writing is a skill. Just like with any other talents--being musical, athletic, artistic, some people are just better at writing than others. That doesn't mean you can't develop writing skills. It just takes more practice!
Many of my weak writers are excellent at math. I create formulas for a thesis, topic sentence, and each paragraph. If you can remember a formula, you can write an essay. I encourage daily reading. The more we read, the more vocabulary and sentence structure we are exposed to. This works its way into the brain, and enables recall when it comes time to write.
I also encourage my students to write about what they read. We do simple things like summarizing, list questions they may have, draw or describe the setting, predict what happens next, or even come up with an alternate title for the book. All of these things help people to become better readers, and that makes us good writers!
You wouldn't expect a middle school football player to play in the Super Bowl. That's what we are asking our students to do, when we ask them to write without practicing, or write without reading. Talent takes us part of the way, but practice takes us all the way!