I don't think there's a clear cut answer. The X-Files was an iconic show of it's era. It also was more government conpiracy than scifi which broadens it's appeal.
But the Fringe story arc was just so well crafted and executed. Time travel and resetting time lines often kills shows (see Heroes), but it was an integral part of Fringe. They also were able to have 2 fairly separate stories, alternate universes and time travel, both be seamlessly interwoven into the main story arc.
This post was edited on 1/23 at 1:26 pm