Secondly, climbing the Rockies on a bike is beyond most people's level of fitness. I would say only experienced road riders would be able to accomplish that.
Thirdly...crossing the Alaskan tundra on bike would be nearly impossible without specialized gear. Standard bike tires would not be able to cross the snow/ice without sinking. Also the consistent freezing temperatures necessitate specialized clothing which is not conducive to riding.
I can easily see why those three landscapes might "stop people from getting around on a bicycle".