If the 10 biggest instances sent out a mail to users who havent been active the last month… we would pretty much be able to answer this question. Until then it will mostly be guesswork. It could also perhaps be a good opportunity to mention the new features.
I am pretty certain the inabillity to connect to irl friends, too much work to build up a base and already being on too many networks are the most common reasons for not staying. I think the abillity to share a group of contacts (like circles on G+) could be an awesome way of quickly get into the network.
The reason I stayed are because of the people I found and connected with. I knew that if I wanted to have a live feed it will not suffice with 10… I need more than 100 or even a 1000… and they all has to have the interests that I like.