As has been mentioned before; the growing bored easily. I fall victim to that quite a lot as well. Another thing that really drains my motivation is simply the way some people write. And I know that's mean, but some websites have this almost collective way of RPing that just doesn't suite me. The site I have in mind while typing this is a Dutch one on which people somehow have this habit of not even trying. Aside from a handful of exceptions that are the only reason I haven't ditched the site altogether, it's not even just oneliners. It's completely disinterested oneliners.
And that combined with most of them not being able to come up with some form of plottwist, I have a lot of trouble keeping RP's on that site, and similar ones, going.

But mostly I just have a really short attention span and get excited for something new far too quickly.