This post might become a rant but I hope it's a constructive one.
For starters as others have mentioned no one thing kills games. In my experience for the most part many are simply failures to launch; early code issues (especially stability or CGen related - if it's too frustrating to create characters players move on), or an unpopular niche theme (requiring people to be familiar with an obscure novel for example).
What I've noticed though, and it happens recurringly, is staff's inability to get their priorities straight.
As a quite real example I've played on MU* before where I was baffled at +jobs about PrP running being downright neglected, sitting there for a week unread or even afterwards staying unsupported, while they pour their resources into creating and maintaining stuff like approved equipment lists. No game has ever died because players sit there going "well if I can't have a Smith & Wesson Model 500 because only that Magnum Taurus Model 608 is available I guess I'll just have to find another place to play!".
Games have died because there's nothing happening and players shrug their shoulders and stop logging on there; they start off well (new, fresh MU* tend to get a lot of folks rolling there to test them out) but then they start leaking activity, and the fewer people remain the fewer opportunities exist for the remainder ones to find roleplay, so they taper off as well.
I don't get it. Plot translates directly to activity and keeping their players active should be a top concern yet MU* get it wrong, and it makes no sense; it's so obvious to see. PrPs scale up, one person can keep 4-5 others entertained and not just while they're running scenes but afterwards too, feeding them things to do, and not that many players run them, or at least stuff more involving than meet-and-greets.
TL;DR: If your players are bored they will leave. Games get visited by many players as they open but they won't stay unless you make sure they continue to have stuff to do.