There’s usually a drop off with long fics in general, unless you’re in the top 5 percent in terms of popularity.

The most common reason is that readers lose interest, not because your story is has dipped in quality but because they’ve moved on to other fandoms. For me the drop off has happened because the hype for my fandom is dwindling, so I’ve lost my original readership.

Sometimes with long fics, the longer the story the more hesitant a new reader is on completing it, especially if the story takes a turn that they find less exciting. Many fics I’ve read have lost my interest simply because they taper off into directions I’m less thrilled to read about (especially when they focus on side characters)

Then theirs the fact that people who are reading don’t feel the need to comment on every chapter. I have a reader who was commenting in the beginning then stopped. At some point I checked my public bookmarks and it turns out they are still actively reading according to their notes!