In the Real world,the behavior of gases only conforms to the ideal-gas equation at relatively low Pressure & high Temperature,Why is it so?

"Ideal gas" is a model. Two basic assumptions of ideal gases are :
(1) The gas consists of molecules of negligible volume.
(2) The gaseous molecules exert no attractive forces or repulsive forces on each other.

In the Real world, the gas under low pressure would occupy a large volume. The average distance between molecules is much greater than the size of gaseous molecules. In other words, the volume of gaseous molecules is negligible when compared with the average distance between molecules. This fulfills the first assumption.

Under low pressure, the average distance between molecules is relatively great, and thus interactions (repulsions/attractions) between molecules becomes very weak. Besides, under high temperature, the gaseous molecules move fast and the time for interactions between molecules are very short. Therefore, the interactions between gaseous molecules become negligible. This fulfills the second assumption.

Therefore, a real gas at low pressure and high temperature is close to an ideal gas.