Why is smoke considered a factor of global cooling when analyzing nuclear winter scenarios, but arent when analyzing global warming scenarios?
The question contains a false assumption that climate science fails to consider the cooling effects of aerosol particles, including from smoke, that are the principle forcing expected for nuclear winter scenarios. Climate science does attempt to quantify every significant effect, both warming and cooling - not only the warming effect from increased greenhouse gases - $\small\mathsf{CO_2}$.



Whilst not as recent as I would like, the graphic shows the principal climate forcings that climate science has identified and includes the cooling affects of aerosols.

Aerosols from burning forests are short-lived, and the $\small\mathsf{CO_2}$ from burning vegetation should be drawn back down by regrowth, whether forest regrowth or grassland. The most significant, enduring climate consequence of Amazon forest being cleared and burned is not the aerosols or the $\small\mathsf{CO_2}$ from the fires, it is from land clearing turning a large existing Carbon Sink into a carbon source. 

Turning the Amazon to savannah would release 200 billion metric tons of $\small\mathsf{CO_2}$. The cooling effect of the aerosols from burning will not persist long, but the raised $\small\mathsf{CO_2}$ levels from the loss of a large Carbon Sink will.