I know part of my problem with longer fics (ignoring that I haven't written even a shorter fic in a while) is that there is a fear of works in progress for a lot of people. I know I've seen quite a few people state that they won't read something until it's finished because they don't want to get attached and then have the writer lose interest or drive or get busy with something else (either fic or non-fic) and end up being stuck. So I'd want to finish something completely and then release it in bite-size chunks.
The trouble there could be that the drive to finish something that huge can be hard to maintain when you're not getting feedback as you go. It may be an epic in your own head but what about others?