Why were the humans of the Paleolithic era nomatic?

You mean nomadic. Nomads have no fixed abodes. They search for food, plant or animal, by following the seasonal abundance of edible plants and the movement of herds of wild animals. The Plains Indians of North America were largely nomadic, although the areas they roamed had limits. On the other hand Paleolithic people, although hunters and gatherers of nuts and berries, lived in fixed sites as shown by archeological finds in caves that must have been occupied by them for consecutive centuries.