A narrow column of text can make it easier to read individual sentences, but it does so by sacrificing vertical space, which makes it harder to skim a page for relevant content and makes it easier for me to lose track of my place since I can't see as much context, images, and headings on screen all at once. I also find it much harder to read text when the paragraphs form monotonous blocks spanning 10 lines of text rather than being irregularly shaped and covering 3-5 lines. I find Wikipedia articles much harder to read in "standard" mode compared to "wide" mode for this reason.

Different people process visual information differently, and people reading articles have different goals, different eyesight, and different hardware setups. And we already have a way for users to tell a website how wide they want its content to be: resizing their browser window. I set the width of my browser window based on how wide I want pages to be; and web designers who ignore this preference and impose unreadable narrow columns because they read about the "optimal" column width in some study or another infuriate me to no end. Optimal is not the same for everyone, and pretending otherwise is the antithesis of accessibility.