Age is a huge factor. Also, and I know I'm among a minority (although probably not among this community), diplomacy and managing geopolitics is among the top of my list when considering presidential candidates. Typically, presidents are able to pass one, maybe two large pieces of domestic legislation. But they manage the global landscape of allies and enemies on a daily basis.
I may just not be familiar with Bernie Sanders views, but I don't feel like he offers much substance on the global stage. I just can't picture him representing and speaking on behalf of the US at a G7 or G20.