Well first of all if you try biking through the desert southwest on a hot day you are likely to die. Imagine crossing hundreds of miles of desert in 115 degree heat. Where do you get your water?
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".
These were examples of what people can do when they set their minds to it.
Of course people won't be commuting to work across hundreds of miles of desert or mountains or tundra. They don't do that in cars either.
There are, however, fairly basic ways of making normal people commutes more bearable on a bicycle: dedicated paths shaded by a tree row, evenly-spaced water fountains, hedge rows for wind-blown snow blocking, reliable snow clearing, perhaps escalator-like systems for climbing up hills along highly popular routes.
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".