It's not just losing interest. Unless you catch it at the very end, when all seasons are produced, each new season carries a startup cost - you'll need to re-acquire a lot of knowledge about the world it's in. (Or, colloquially, "Who the fuck are those people and why do I care" ;)
At some point, it's too much. It's becoming work. And if you learn/relearn an entire world, why not something you haven't seen before? So the series gets ditched.
At some point, it's too much. It's becoming work. And if you learn/relearn an entire world, why not something you haven't seen before? So the series gets ditched.