It's more of a guideline than a rule. You CAN sell a show into syndication at any time, but are not likely to get a good monetary return unless there are a lot of episodes in the bucket. Syndication is usually a five-day-a-week run and the networks would prefer not to be resetting to episode 1 sooner than four or five months. That's why shows like Cheers, Frasier, MASH, Friends and The Big Bang Theory end up being so popular in syndication. They ran for at least a decade and it takes forever to hit the reset button on them, even when TBS plays them in five-episode blocks.
As for why Northern Exposure doesn't air? The episodes are self-contained, so there's no problem there. I'd say it's because it's a niche-specific show that hasn't shown to have a lot of demand. Procedurals are popular, reruns of Law & Order have always done well. A show about some schmo in Alaska? Can't see a lot of people wanting to revisit it. Some do, I'm sure, but I know that I personally would not watch it if it were on.