As a scientist and teacher, I was often confronted with the task of communicating very complex ideas to people who, while intelligent, did not have all of the relevant information necessary at the forefront of their consciousness to understand the concepts I was trying to convey. For that reason, scientific writing strives (not always successfully) to be as clear, simple and concise as possible. I was fortunate to have good teachers – that article was required reading in our lab.
One of the (many) problems with scientific English is that so many non-native speakers publish in it, and they bring a lot of baggage to it from their native languages. But the main barrier to understanding is that scientific prose is that it is dense with new ideas. If you do not know the precise definitions of the terms the author is using, you will be lost, no matter what your level of skill. If you have not worked out the math before, you will need to do that when you encounter an equation, or the words that follow will make little to no sense. For this reason, simple, declarative grammar is the byword for a scientist – the ideas make things hard enough as it is.