Why is a single walled pipe not used for wood stove exhaust pipe on upper floors?
You have to use double walled pipe when going through walls and floors and what not because the double wall and the space between the pipes cools enough that it can come in contact with wall materials. Single walled pipe would get hot enough to ignite the surrounding materials. 

Also, a lot of fireplace type units use a 2 walled pipe that brings in air for combustion through the outside pipe and exhausts hot air through the inside pipe, but that wouldn't be used in conjunction with single walled pipe.