Having RPd on the internet for a long while, and for a fair little time here my answer would be the normal life expectancy of any RP is measured in butterfly. Anything that goes beyond that is a rare thing. I dont generally believe there is a "reason" for it very often, well there is but its not a direct coloration to the RP it self sometimes. I have had RPs die both from my partners and my side because we started and some shit happened in real life and it just kind of never went any further. Heck i had a couple of RPs die semi recently on post 0 its not anybody's fault. One potential writing partner got really busy suddenly the other just had one of those mental blocks.
I have ended RPs before and had RPs stall because while i might have a vague idea of the next step it just dos not want to congeal in my head so it sits there for a while, i will send a PM to let my partner know whats up. Sometimes your brain fires up again sometimes it dos not, in the same way sometimes you write a few back and forth with somebody and you come to the conclusion you arent really working together *Shrug* it happens.
Personally i have found honesty the best course, write the shitty PM nobody likes to write say "Hey this that the other not working, well crap, best of luck dear John" its not really anybody's fault, what i would suggest take a little more time in PM prep talk about the idea and the characters usually in the back and forth you can get a better feel for the other person and you can find out more closely what they want, and if its something you want to.