Soros regularly uses his wealth to pursue political aims throughout the world. He often does this through "philanthropic" groups. For example, UK news stories have been popping up about him spending £700,000 on anti-Brexit movements. The Koch brothers might be a similar comparison?

I think that they become bogeymen because their wealth provides substantial power and they're involved with controversial things. On the other hand, people like the Gates focus more on "dry" issues. (Edit: or maybe they have better PR!)