IGCSEs are International GCSEs. They are designed to be taught anywhere in the world, in any circumstances, with no resources at all. You can do it with a teacher talking and a pen and paper, in a refugee camp, in a war zone, in a secret underground school, in the middle of the desert, without electricity, without a photocopier, without the internet, etc.
That might not be how they are taught in wealthy countries like ours, but everything extra is optional.
Whereas GCSEs require lots of resources, science experiments, history trips, media clips, work sheets, technology, tools, materials, food, internet access, etc.
iGCSEs are cheaper to teach, and also a bit easier, which is why private schools teach them, and state schools are not allowed to!