Unfortunately, most of us grew up. We were kids when we played every day, and now we're adults with jobs, college, families, etc. and don't have time to play games like this as often anymore. Some people moved on completely, but there are others that have lingered, but just can't get on as frequently as we used to. While new people still join every now and then, its hard to keep them in the community because the majority of the original player base have moved on or just no longer have time. For a long time it was hard to gain any new players because the register button would only be available for small windows of time on random days.
Honestly the inability to register for the longest time probably killed the community more than anything else, with a lack of incoming new members, only the people already registered could play, and therefore, as they (myself included) aged out of playing the game and entering the "real world," the community started to falter. If the register button hadn't been so closed off, the community would probably not be as quiet as it is, as there would've been a more regular influx of new players that could help keep things going strong.
It's a shame, but we can only continue to try to keep it alive.