one simple reason would be nuclear deterrence. None of the major powers wants to start a war that involves their major population centers or those of their allies. Hence many modern conflicts are being fought in third world countries and other regions that are not vital to super powers.

That's a simple answer. THere are still counter examples such as the siege of Sarajevo, Baghdad saw fighting in the recent war, and the Ukraine has seen some fighting. But these conflicts dont seem to involve a confrontation between super powers so perhaps it supports the thesis in my first paragraph.