I would guess that's not _that_ low that it would have made an impact on those reading those reviews.
Chances are this has more to do with the limited, high-brow, circulation of these reviews. Harper's, for example, seems to have had a circulation of around 50,000 in 1851 (https://en.wikipedia.org/wiki/Harper%27s_Magazine#History). The USA had a population of about 23,000,000 at the time (https://en.wikipedia.org/wiki/1850_United_States_Census), so it would have reached less than 1% of the population.
Are... these reviews considered difficult to read or especially overwritten, then? Or did you mean something else?
> bad rhetoric, involved syntax, stilted sentiment and incoherent English
Are these not legitimate grounds on which to judge a book? To the extent that these are smaller concerns for modern reviewers (are they?) I'd guess it has more to do with our having settled on a style of prose that allows little variation and is very simple, for a large majority of literature. We now expect and receive little poetry in our prose, to put it another way.
Language has become Hollywoodised over the last few decades. We've lost the ability to express colour and imagination through language - not just through plot or predictable genre tropes.
Modern fiction is more like a journalistic description of a movie or TV show, and less like an independent medium that sculpts language into story structures for its own sake.
All interpersonal communication in any medium is semantically lossy- language is an intermediary through which you are attempting to encode a concept or story or feeling, so that someone else can decode your words into a similar concept/story/feeling whatever inside their own head. Language is the middleman, and an author's skill with language imposes a ceiling on how well they can transfer their ideas to other people.
In the case of fiction, language serves double duty as an art form in and of itself. Both the medium and the message are components of a creative work. Otherwise, every novel would be nothing more than outlines of plots describing sequences of imaginary events, which would be pointless.
I would also disagree with the characterization of /r/iamverysmart- in principle at least, it isn't there for shunning people trying to sound literate. It's for teasing people (usually teenagers or college undergrads going through a phase) who have started to learn interesting things about the world, who are very impressed with their own knowledge, and are expressing ignorance and narcissism rather than the 10-megaton knowledge bombs that they think they're carpet bombing everyone with. Not that I'm in favor of shunning anyone who isn't harming others- it's too mean-spirited and easy to abuse.
Not all that low. Even in 1870, 80% of the US population were literate (and nearly 90% of the white population).
So there was a lot of criticism of the superficial aspects of the writing - "bad rhetoric, involved syntax, stilted sentiment and incoherent English". The words and the "English" are excessively focused on.
Today we take literacy for granted, and anyone who tries to sound more literate is shunned (/r/iamverysmart).