I saw a discussion about this once and they argued that they didn't have the range leaving all the rural people still around. Plus the humans who are in nuclear proof bunkers/submarines have nukes to get revenge when you land.

Blocking the sun works better. You can't just hide from the earth freezing. Even if you survive somehow you are giving away your location via infared. Also, you could still have some functional ecosystem left with the plants and animals that can withstand a true hibernation and the deep sea life.