The majority of cases I have witnessed or experience regarding game death can be cut down to two major, critical components. These two elements are relatively clean, solid deathblows to a roleplay and they are complacency and excuses. Both are detestable things in a social game, inherently poisonous. While there are other factors outlined above, my experience has always been that other players become complacent and or then develop excuses.
What do I mean by complacency? Say that a topic has a new post in it and there is something a character can reasonable do about it or interact that would in some way further the story and the plot, even individual plot and development. Unless it would be all fluff for the sake of posting why not post? But complacency is not just limited there either, it incorporates the issue of having a post due by a player in some time and them repeatedly putting it off, telling themselves, "Oh, I can write that later." or "I don't feel like writing now... tomorrow then..." This becomes a habit of procrastination and does nothing but infect the topic with that mindset. When is a tomorrow that never comes? One day becomes two, two three, four, five and so on. Some players may leave you behind, but generally people become complacent themselves when they bear witness to complacency and that others might excuse it.
What comes next is a component of the never ending issue of procrastination, in the form of excuse. Everyone on the forum and the world at large has some extent of a life to attend to, one good, bad, or mostly likely somewhere in between filled with trial and complication. We all exist in this world and we all are, by the numbers, adults or young adults. Excuses are a form of social token or currency, something dropped into the awaiting hands of others when something promised to be done or realistically due is not. Generally people accept them, because in turn they produce these coins themselves and hand them off; some have more, some have less, some accept more, some render more. The issue with this is, is that everyone has them. They are plentiful and thus devalued, meaning that an excuse is little more than a hollow admission one did not budget sufficiently or extend the effort or time to compensate for a shortage of opportunity.
A post takes anywhere from some fifteen minutes to at most, absolute most realistically, an hour to write in one of the most advanced sections or topics. For the majority, those casual, I believe it is fair to say a post will take only some fifteen to thirty minutes if one sits down and dedicates to it there and then. It might not be the best work, it might even be subpar for your standard, but roleplaying is a social exercise. People have lives of their own and expectations, failing this has greater consequences than the self. In fact, I would wager the sheer amount of complacency and excuses, most infamously of "I was busy.", to be the major negative factor in most topics if I were one to bet.
However, that is just my experience and I have a very low threshold, abysmally low one, for tolerance with complacent people or the excuses of man. I do everything in my power to strike back against it, to the point I will tolerate and turn blind eye to other issues that are more minor so long as it keeps the activity going. Though ultimately those are the two consistent killers of topics I have met first hand.