Possibly they don't! Several other people at the book club complained about the historical accuracy (of the story itself), the characters and the pacing, but I was the only one who said anything about the prose. I think a lot of it is just that most of what I read is not modern, so especially modern words and phrases really stand out as 'hmm, I don't often see that in books, that's distractingly strange'—and deliberately trying to write in an older style certainly helps, yes!

(Really, I'd put it the other way round—why shouldn't a humble amateur fanfic writer, who puts effort and attention into understanding and imitating period style, be really good at it, regardless of how bad professional authors can be? :) )