Why it happened: The website went down for a good chunk of time some weeks ago, and that had a few others fall out of the habit. Otherwise based on what's known and sparse activity from some it's easy to imagine who's reduced themselves to just lurking (I know who you are).
What's happening now: Clearly BR, much like when you thought SC4 was "The best SC has ever been", you're projecting your own views of the place and assuming it's how everyone feels. This isn't unusual at all for our group, and the number of members during your boastworthy period isn't really too different.
Remember when you thought the website was dead, and that Boards.Zone was going places... just because no one would kiss your ass?
What we can do about it: If your perception of participation here is really that contingent on your own participation within it, all you need to do to feel like the place is more poppin' is to contribute more to it yourself, right? Otherwise I'd recommend trying to bait some out of lurking.
The Weather Argument: Our more active times tend to be Spring and Fall anyway, when all our energy's spiking and we need somewhere to throw it all. It's not about who's home for the holidays, as a lot of people use this website mobile-y or at their place of work, it's about how much drama is happening in our own lives that we can use as a resource. Summer and Winter are fairly dead times from people not just being busy, but tired from the weather.
Seriously, if I had a dollar for every "SC is dead"... it's beyond the realm of Necromancy at this point.
I contribute about at the same rate I always have. And yeah I could bait but that bait may only work for them replying to one post.