Well, I can attest to being on both sides of this, watching people disappear, and being the one who disappears. 

I can't say why other people drop out, but personally, what happens with me is that if I'm interested in a lot of stuff going on all at once, I'll sign up for something, or express interest, and then later forget that I did. 

I've been getting better at this, but it still happens at times. I'm really pleased with the whole 'subscription' method, since it means that I'm alerted whenever something happens in those threads, reminding me to check them. But it also means that if I forget to subscribe, it's likely I won't think about them when I'm doing the rounds when checking my subscriptions.