Well we had to kill Crota. He was a direct threat to earth. Oryx only showed up because Crota was providing him with so much tribute, when Crota died that tribute was cut off and his worm grew hungry. He couldn't care less about Crota as far as father/son is concerned. Hell, he threw him into the vex portal and told him to fight his way across the universe.

Crota was sent to search for the traveler and devour it's light. When Crota found it Oryx was already on their way.