We each have our strengths and weaknesses in whatever we do. There are some who are like you, some who are the opposite (where they are bad at math and good at writing), some who are good at both, and some who are bad at both. There really isn't a reason as to why we are naturally good at one thing but not another. So to answer your question, I don't know why you are like that and no one will ever truly know why.
However, that does not mean we should simply give up at the things we are not naturally good at. Writing essays and research papers takes a different approach than with solving math problems and doing proofs. Unfortunately, I cannot tell you exactly how these approaches differ because I can't guarantee that we even have the same approach when it comes to doing math, let alone in converting it to writing. This is something that you have to figure out on your own in order for it to make sense to you. In order to get suggestions though, you should try to speak with someone who is good at writing or you can consult text resources either in book form or online. There are many different types of ways to think about writing and you just need to find one that suits you. And of course, you should always try to practice writing by making drafts of essays and have people critique each draft. Just learning the theory and method of writing won't get you any far if you don't put them to practice.
