"It handles large documents efficiently" - and plain text doesn't? That's just an insanely bad reason to use word. Vim can open, edit and "handle" very large documents with ease. A typical novel - even one by Thomas Pynchon is trivially small for plain txt. At roughly 300k words for a long novel, 400k if you're Tolstoy, that's tiny.
Spell check maybe is a reason, no writer wants to admit they can't spell "Hemingway", maybe word/page count abilities, but large documents isn't a reason to forgo the benefits of plain text.
Most of the world doesn't want to deal with Vim's normal vs insert modes. Nor do they want to engage in a philosophical battle about proprietary formats vs plain text. Most people just want to get stuff done, and in this case, send their work to other writers in a recognizable form that they can easily read, edit, make comments on, etc. Word and other programs offer that to them. Plain text and Vim do not.
Spell check maybe is a reason, no writer wants to admit they can't spell "Hemingway", maybe word/page count abilities, but large documents isn't a reason to forgo the benefits of plain text.