Time of year is definitely a factor - as summer winds down, people get busy either going back to school themselves or with their kids going back to school. I don't know if reviews have decreased over the years, but I do agree with Mare that it's a different batch of people doing the reading and reviewing.
I know personally, I've averaged way fewer reviews per chapter on the story I'm writing now than Curtain Call last year, but that doesn't surprise me at all given that it's an AU and not about Nick. It doesn't have as wide of an appeal. I think AU in general is still a turn-off to some readers, no matter which Boy it's about.
When I compare the amount of reviews CC got to the amount of emailed feedback I got for Broken nine years ago, it's about the same, but the people giving that feedback were different. The reader base as a whole has probably decreased over the years as the fan base has decreased, but we also have newer readers who weren't reading fanfic ten years ago to replace some of the ones who have lost interest in it.