Not to mention the premise of the episode makes no sense. No one bothered to check in on this colony... ever?
Star travel wasn't easy before the Warp 5 engine. It would've taken months to get there, and with no certainty of what they'd find, maybe it wasn't deemed worth the risk. Although I suppose they could've asked the Vulcans to find out.
It's easier to go under ground and live there than to just... walk away?
Spending too much time above ground would kill them. A large portion of the planet was irradiated. They couldn't have walked far enough before they succumbed to the radiation. Remember, they had to be transplanted to a whole other continent on the other side of the planet. Not exactly within walking distance.