I suspect having a smaller number of longer chapters, relative to some other stories that have similar word counts, probably reduces the number of reviews, since each chapter attracts some and some reviewers post one for every chapter they read. Notice the relevant stats for the five stories ahead of NoFP:
Harry Potter and the Methods of Rationality by Less Wrong
Chapters: 116 - Words: 630,572 - Reviews: 28715
Harry Crow by robst
Chapters: 106 - Words: 737,006 - Reviews: 24440
Partially Kissed Hero by Perfect Lionheart
Chapters: 103 - Words: 483,646 - Reviews: 14632
An Aunt's Love by Emma Lipardi
Chapters: 71 - Words: 356,847 - Reviews: 14358
The Marriage Stone by Josephine Darcy
Chapters: 77 - Words: 381,147 - Reviews: 13553
For that matter, most of the stories
below NoFP in the top 25 by number of review also have many more chapters; only four have fewer, and they're not far behind, with 36, 36, 34, and 32 chapters, and each of those has less than 10K reviews. If we look at the ratio of
reviews per chapter, it looks like NoFP is a runaway first -- nothing else even comes close.
NoFP's only really close competition for favorites and follows is "Harry Potter and Methods of Rationality," which is also #1 in reviews. That one might pass NoFP in favorites before Matt finishes Chapter 41 -- it's going to be complete at 120 chapters as of next Saturday, and I suspect some folks who were holding off reading it while it was a work in progress will be persuaded by their friends (like me!) to dive in once the final chapter posts up. It shouldn't pass us in follows, because as a finished story there won't be any more updates to follow, but then
my own finished work is still picking up follows from time to time ten years later -- some FF.net readers are kind of clueless as to how that works.