Why are the citizens of G8 countries responsible for a disproportionate amount of greenhouse gas emissions?
Because these countries are the wealthiest in the world and this wealth is largely dependent on burning fossil fuels.
Burning fossil fuels and economic growth in the G8 countries have been strongly linked over the past 50 years or more. Fossil fuels are the main source of energy and power economies from electrification, cement & steel production, manufacturing and transportation.
Developing countries either do not have sufficient fossil fuels, or the money to pay for them, or the infrastructure to utilize them and so their economies have been sluggish or slow to develop.
However, under the new UN Paris agreement reached last December all countries have agreed to reduce their greenhouse gas emissions, so the world is going to have to reduce their collective use of fossil fuels while not letting their economies drop.