There are, honestly and all kidding aside, a number of factors, I believe.
One is going to be the definition of "constructive criticism." Some authors think that anything that tells them their story isn't perfection is a flame. They do this for the lulz, and not necessarily to improve, and those readers who like to encourage authors to improve on their craft and offer honest, thought-provoking feedback aren't welcome to such writers.
On the flip side, there are those people who think they're being constructive, when they're really serving to be destructive in the terms they use to tell the author there is a need for improvement.
Another factor is the "something for nothing" mentality. It permeates our society, so it isn't really any surprise that readers wouldn't think twice about not letting an author know that they read the story. Liked or not, they read it. I tend to think of it this way: "hits" are the number of times someone checked the book out in the card catalogue, read a few pages, and put it back on the shelf. The "hits" mean nothing to me.
The reviews, on the other hand, tell me quite a bit. They can tell me that the story was good, that it needs improvement, that it's total fluff, etc. As I am one of those who doesn't write, I can tell you that I also look at the names of the reviewers. If I recognize the names as people that read fluff fic, whose names I only see reviewing fluff pieces, and they rave about the story in their review, I'll generally leave it alone. I'm not usually in a fluffy mood. There are those rare occasions when my brain can't handle much more than fluff (say, when I had the flu), and those are the times I'll read those pieces.
The 'least loved' stories in the archive - but the most read - are generally oneshots. People read them, and -let's face it- 'fap' to them, and close the browser window. Sometimes, the readers leave TMI reviews (IMO) but at least they're giving the author some feedback about what was written.
I can honestly state that another reason readers may not leave a review may have to do with their age. DG, at the beginning of this year, had to do a massive sweep through the archive to nuke underage reviews that weren't getting deleted when the accounts were being removed for minors (details here). Being completely underage aside, younger adult readers won't necessarily leave one either. They're not at that point yet where they realize that the authors don't have to write - and even if they do write, they don't have to post it for anyone to read. Sharing it is completely up to the author of the piece.
I'll be completely forthright: I'm appalled at the lack of reviews. I understand not having a great deal of time to leave a massive review - and most authors don't want that anyway. They just want "I liked such n such" or "I didn't like thus n so" and a reason as to why a reader felt that way.
But one thing that I think keeps a good number of people from reviewing a story is not knowing what to say. Particularly if it's a very well-written piece. Readers tend to think that on a completed story, or on something that's by well-loved authors, they can't offer feedback. (Especially if they didn't like something that well-loved author did - heaven forbid you read SS/HG 'ship in HP and not like an aspect of a story by one of the big names that write in that genre.)
The bottom line is that I wholeheartedly agree - reviews support the authors. Sadly, a lot of authors don't have tough skin and throw tantrums in their author's notes. (Yes, we've seen this - I see it on cleanup quite a bit.) Not knowing how an opinion is going to be received can sway the choice of the reviewer to leave it. Just as all of the authors in the archive take a chance on putting their work out there for scrutiny, the reviewers take a chance on leaving their opinion of the work.
But I think it's a chance that should be taken, given that the author put him/herself out there first. All a reviewer is being asked to do is respond to the author's effort. After all, if you took the time to read it, telling the author you did so won't hurt.