Why Paleoptera can't fold their wings?

Paleo in the name Paleoptera means older or more ancient. The insects grouped under Paleoptera are thought to be more ancient than the other insects which have wing-folding mechanisms, known as the Neoptera. Neo = new. Because the Neoptera evolved wing-folding mechanisms, it sets them apart from the Paleoptera. It is therefore thought that the Neoptera's ancestor(s) is(are) insects in the Paleoptera and the Neoptera evolved wing-folding meachanisms from ancestors that did not have such mechanisms. Without these mechanisms, the wing cannot be folded.