Just trust me when I say things don't always follow a predictable narrative path. Ultimately, we obviously can't account for every character's motivations and desires. That's the nature of group roleplays; you either give people the freedom to make the characters they want but give them at least a passible reason for sticking together, or you heavily restrict people to force them into the setting at the risk of alienating them and making people play a character they don't want to. We went with the former; we want people to play the characters they wish to, and in exchange we ask that the players suspend disbelief enough to keep your character with the narrative.
I mean, we could have said that everyone is a bandit in the same group who got arrested, but that limits player freedom.
The problem with characters breaking off is if we let them go their own way, we suddenly have to make an entirely separate plot with a bunch of new NPCs and account for continuity between each of the story arcs. You can see how this would become a nightmare; one person goes, and then another, and another, then you have a bunch of players on their own not interacting with anyone and then get bored and quit, thus nullifying any reason for us as GMs to work on each separate narrative.
Another reason is practicality; players drop out of all roleplays, it doesn't matter if it gives out free blowjobs and a micky of rum every week, people will leave. Players start branching off, and suddenly you have different groups going at different paces and then one group gets stuck because one or more players start going slowly for posting, or they drop put and leave a narrative void. It's a stupid mess.
Ideally, nobody would drop out and we can all contribute to the progressing narrative, but hard earned experience has taught me that you can't think of any character as being irreplacable to the plot and you can't pigeonhole players into special roles.
One kind of RP that always gets screwed are the ones where every character represents something (let's say Zodiac signs), and there's a prophesy that these characters are all needed to save the world or whatever. A month passes and the player for Aquarius drops out because she took on too many RPs, and the player for Gemini dropped because of school. Now the GM is struggling to find replacements, and at the same time the players for Aeries, Taurus, and Cancer aren't posting at all, and making excuses.
The RP is suddenly VERY screwed. Once it goes into that nosedive, there's such a small chance at saving it.
This is why I took on more players than I strictly wanted for the RP (I tend to like groups of six), but I wanted to give more players a chance to participate and to have a buffer in case people do drop out. If I had six, and three people leave this month, I have three players left.
Please don't take this as me being snippy or anything of the sort, I'm just explaining how we run things for these games, and why we set things on a mostly linear narrative (that and roleplays honestly take eons to accomplish any story arc; case in point, my Mass Effect RP is just wrapping up a mission now that was literally "clear a small mining facility" and it's been over 2 months now), so the more we streamline it, the sooner we can get to the good stuff.
So, yeah. Hope that clarifies the method to my madness and why I generally have to beat away players with a broom because the games generate a lot of interest.