The author of the standalone novel on which this series is based also wrote a series of novels in which it is revealed that all his standalone novels are, in fact, interconnected.
This novel was adapted to a well-received movie decades before it was adapted into a pretty darn good tv series. It ran on cable, not broadcast tv.
The premise really did lend itself to an episodic approach, though the original novel and movie didn't really get to exploit that.
Producers predicted a seventh, final season, but they [and fans] were disappointed by fate: the network's decision to cancel after the sixth season finale, which ended on a cliffhanger that was never resolved. Sure, you could read the book or watch the original movie to get the original ending. But the artistic freedom to take the story to a different place than the foreordained ending was eliminated, sadly.
On the bright side, World War III hasn't started.
Hallelujah. Hallelujah.