I think time is the essential killer of peoples motivation to be a part of a RP. As seasons change so do peoples lives and their cravings. It is best to give a roleplay a high point of accomplishment to feel completed. It also depends on the roleplay really. As [@Odin]said open world sandboxes have their own issues and I'm sure the others have their own reasons dependent on their style. 

But for me I run a continuous open world "sandbox" you could say. Through trial and error I'd say its definitely a passage of time [i]without[/i] activity during a roleplays life. The beginning is the most hyped time of a roleplay and peoples motivations are high. Without constant story progression I've noticed people have been likely to drop out. 

Progression falters seriously from the GM not being available. It falters a bit when a player is not available. Most of the time people join when they are available. Only after some time has passed does their schedules change. So keeping a roleplay from being inactive while also having it progress in a steady interesting way is key to keeping it going. The opposite is why it usually dies. IMO