Because “big lists” suck on Stack Exchange. If a question calls for one item per answer, then you get a large collection of mediocre answers. Older answers get to have more votes and newer answer remain largely ignored: voting reflects pretty much only age, also popularity if you're lucky, but forget about quality.
Good answers are comprehensive. They discuss the relative merits of different approaches, the different environments that could lead to different scenarios. They seek to cover the problem as a whole, not to exhibit one tidbit. If a question calls for that kind of answer, it isn't a “big list” question.
If something calls for a big list, it's better suited on a wiki, such as TV Tropes.
I once thought that list questions were fine. And then I saw them at play on Science Fiction and Fantasy. (You can see our best one preserved for posterity.) And then I happily burned the lot of them.
Questions like “What are the issues with faster than light travel and how might they be overcome?” are too broad. You could write a whole book about that. More focused questions, about e.g. what the irruption of FTL technology would do to a particular society, or what FTL technology could enable a particular plot, are perfectly fine and welcome here.
Oh, and the tag itself would be a meta tag: it doesn't help to categorize questions in any way.