That comparison between the readability of Newton and Darwin has got me thinking on what changed between the 17th and 19th centuries to make that possible. Aside from the obvious differences between how Newton and Darwin communicated in writing (Latin vs English) I think it also might have to do with the rise of the novel, and how novels effectively created a "general interest reader." I suspect that Darwin and Huxley were intentionally writing for mass reading public, people who read Dickens and the Illustrated London News. These sorts of people wanted to learn new things about the world but wanted things to be readable and memorably expressed.

Newton was basically just trying to communicate with a handful of experts, many of whom didn't even speak English, so he just didn't see what he did as something that demanded a lot of attention paid to prose style. I actually find the writing style of some of Newton's contemporaries to be really beautiful and readable (Margaret Cavendish's Observations upon Experimental Philosophy is a surprisingly fun read). As is some of Descartes (like his Meditations). But those are definitely outliers.