I read this story a while back (while it was getting written), and I actually very briefly conversed with the author by email.

In short, the reason I think it is so popular, aside from the fact that it is so well written, is that it embodies some element of realism. (the author in fact told me, he strives to do this as he writes).

In many stories I read on the web now, things 'work out' so ridiculously easily for the protagonist, that I can no longer connect with the story. The story essentially becomes a list of heavy scenes with little to interest you in what comes next.

In this story, things happen in such a way that it is at least border-line believable; i.e. not all submissives immediately enjoy their lot, etc.

This story is not a manual for an 'ideal' BDSM lifestyle; and speaking more from my own opinion I actually find this a very positive thing.

In any event, once a story gets on to the top 10, and is one of the first you see when you click "recent pop.", the number of viewers skyrockets.

Just my humble opinion.