Complex sentence: Steven Spielberg wrote and produced, but had a clause in his contract to prevent him directing another movie while he made "E.T. the Extra-Terrestrial" resulting in Hooper directing based on his work on The Texas Chain Saw Massacre.
Split into simpler sentences: Steven Spielberg wrote and produced, but had a clause in his contract to prevent him directing another movie while he made "E.T. the Extra-Terrestrial". Therefore, Hooper was hired to direct based on his work on The Texas Chain Saw Massacre.