I like Jeff's idea for a jamjar explanation, but I'd also like to chime in that I prefer the not-fully-jamjar setup to the forced-imprisonment kind of thing.

I understand that some characters struggle with the "why would they willingly stay here" motivation bit, but with a jamjar setting, I think some character types would struggle as well, since realistically some folks wouldn't be all that okay with being forcibly stuck on an island, but there wouldn't be any kind of progress they could make on what would be a huge part of their motivation -- escaping and going home. Plus, I think completely cutting out the world-hopping nexus stuff would be more of a limiting factor than a helping one.

Maybe a system that's only mostly-jamjar or something? I admit I like the monkey's-paw style idea mentioned above where players earn the right to leave (or think they've earned the right to leave but monkey's paw), so there's lots of room for players to say their "I don't wanna be here" character has had terrible luck with actually getting home while other characters can use the world-visiting for plots, etc.?