How to diagnose problems in a paper rejected in pre-review?
The pre-review stage is generally a very quick assessment of the manuscript in which editors go through it to check primarily whether the article matches the scope of the journal, the level of novelty of the study, and how interesting it would be to the target audience. For a journal that has a 70 percent rejection rate, I don't think they would give enough time to a paper at the pre-review stage to reject it on the basis of the writing style, unless the style is really very bad. They would probably just read the cover letter, abstract, and results and discussion section carefully and skim through the rest of the paper.

I don't think there is any way to learn more about the reasons for rejection from the journal. However, don't get upset or think that the quality of your paper is not good. Since this journal has such a high rejection rate, it is natural that only the very best or the most path breaking studies will find their way to this journal. This journal might not consider your study novel enough or interesting enough, but there are many other journals that will find it interesting. At this stage, there isn't much that you can do to improve your paper, as you haven't received any reviewer comments. You will have to just submit it to the next journal and hope that the paper goes through peer review.