It's not just economics, it's logistics. Countries like Bangladesh don't have the space to build wind and solar, and they don't have the sophistication to manage long distance grids with batteries, EVs, etc. That's why they keep building more coal. They're not stupid, there is just are practical concerns these countries confront that don't exist in countries that have more central organizational capacity.
And the math just sucks. Say countries like India industrialize with a high percentage of renewables, etc., so they end up with developed economies at just double the current CO2 output of India (4 tons/capita). That would mean that the growth in CO2 output from the subcontinent alone (India+Pakistan+Bangladesh) would exceed the current CO2 output of the entire EU.