Why is it unexplored?

One thing that bugged me a little about Western Marches as described was that there wasn't a really clear reason for the wilderness to be unexplored.  Sure, it was dangerous, but it didn't sound dangerous enough to stop a motivated army, or a mass migration.  So, some thoughts on reasons the land you want your PCs to explore hasn't already been settled.

• Hostile natives
• As mentioned above, I feel that if an area has sufficiently hostile natives to stop exploration and colonization, they're probably too hostile for small groups of low-level PCs.
• Taboo
• The land is off-limits by decree of religious authorities.  I like this one.
• Pros: Lack of other explorers and reticence of henchmen and mercenaries is totally explicable without raising threat level.
• Cons: None of the merchants will want to sell the PCs anything but that's really a benefit because The Game Is In The Wilderness and you taboo-breakers aren't really all that welcome in town.
• Cursed / Haunted
• Sort of like taboo, but with some teeth behind it.  May also not actually be haunted; in this case it would be like taboo but without the ostracism.
• May entail some increase in danger level, but also provides a glorious excuse to deploy more undead, supernatural monsters, and magical locations.
• Ex: Judge Dredd's Twisted Earth, Chernobyl Exclusion Zone, Wheel of Time's Blight.
• Hadrian's Wall
• The natural response of a decaying empire to Hostile Natives - throw a wall up, they can do whatever they please on that side.
• Provides a very tangible boundary between civilization and wilderness.
• Unfortunately very in vogue at the moment, what with the Game of Thrones.
• Literally Undiscovered
• The island nobody's found yet, the bowl in the high mountains no climber has yet reached, the promised land where it rains milk and honey on the far side of the desert of mulebones...
• If there's actually a settlement close enough to supply the PCs, though, someone else probably would've found it.  And if not, they're going to be doing some really long slogs between adventures (as we saw with Scaled Continent).
• Cataclysm
• A catastrophe has torn the land asunder.  It's scary out there, and all our maps are wrong.
• Unfortunately, this sounds like something a competent army would put scouts on immediately.  Unless the army were busy suppressing riots in the capital, I suppose.  But in that case, there is an urban game to be had.  The other case is "the land has been torn asunder and we're right in the middle of it, and have no idea if the next town over is even still there.  There is no army, besides the garrison in the tower, and they'll be staying here thanks."  Problem: where do new and replacement PCs come from?
• Exodus
• Something has forced an agrarian population to abandon their settlements and march into the wilderness in search of new lands.  Autumn has arrived and preparations are begun for winter in a temporary camp.  The PCs, restless youths that they are, shirk their duties and take it upon themselves to go exploring.  In the spring the settlement moves, probably to a location they have found and deemed safe-ish.
• This lends itself nicely to a 'plum pudding' of danger, with safe and dangerous areas intermixed rather than a hard gradient.
• Would make a good setting for a neolithic game.  The tribe has agriculture but not metal.
• Suffers from the same problems with new PCs that Cataclysm II does.
• "Logistically Intractable"
• Some combination of hostile natives, local diseases, hazardous terrain, bad weather, and brutal summers and winters make mounting prolonged military campaigns or sustained settlement here very difficult, but exploration might be workable for a small, motivated group of PCs.
• Probably the most realistic option.
• Not likely to be completely unexplored; observe the occasional abandoned homestead.  Perhaps there is a map inside; perhaps it's full of undead.  Perhaps both.
• Examples: Russia, Africa.