Because it doesn't cool the global temperatures by virtue of being itself. Nuclear winter is caused when massive amounts of debris and other particles (like smoke) are shot up into the air, blocking some of the sunlight, resulting in cooler temperatures. This also has the affect of less crop yields due to reductions in sunlight, less electricity generated from solar panels, and other consequences. So unless we want to voluntarily eat significantly less and use less energy in the name of cooling Earth's temp for 5 years, it's not a very viable alternative.
