Because not all countries have the right temperatures and conditions for that particular sporting event. For example you can't have a snow boarding event in the middle of a really hot country because there would be no snow ,and the weather would simply not be right for the sporting event.
The Summer Olympic games were hosted by Munich, West Germany. The Winter Olympic games were hosted by Sapporo, Japan.
Munich hosted the Summer Games and Sapporo hosted the Winter Games.
Edmonton has never hosted an Olympic Games, either Summer or Winter. Salt Lake City hosted the Winter Olympics of 2002.
Mexico City hosted the Summer Olympics and Grenoble hosted the Winter Olympics.
Norway Winter Finland Summer
Austria Winter Japan Summer
Austria Winter Canada Summer
Switzerland Winter Netherlands Summer
Germany, Canada, USA, France, Italy and Japan. Russia and South Korea have held summer games and are scheduled to host winter games.
Tokyo, Japan hosted the Summer Olympics and Innsbruck, Austria hosted the Winter Olympics in 1964.
Switzerland Winter United Kingdom Summer