As I understand it, Fox approached Carter because the 24 thing went over like gangbusters, Carter approached both Duchovny and Anderson, and Anderson said she'd do it if they could find a way to fit in her schedule, hence the six-episode miniseries (I think Carter was originally wanting nine or ten episodes but it was just logistically impossible).
Ah. Like last time with IWTB. Only difference being that it was a writer's strike.
Yep. I think they wanted more too, but it just didn't fit in with everyone's schedules.