So that people in poorer countries can use more of them in our place to industrialize and then join everyone else in the gangrape of our environment!
Fossil fuels are the cheapest energy source because of they are already established (economy of scale) and are subsidized heavily. In a world were all major economies have shifted to nuclear power, solar power, and wind power, that will no longer be the case. It will be cheaper for developing countries to use green tech.
Also, once the powerful countries have moved off fossil-fuels, we'll hypocritically force poor countries not to use them anyway. Come now, be reasonable.
Or have I misunderstood your comment?
Use less fossil fuels != kill a lot of people :) Not in the western countries, which waste them left and right.