6 Now Esau learned that Isaac had blessed Jacob and had sent him to Paddan Aram to take a wife from there, and that when he blessed him he commanded him, “Do not marry a Canaanite woman,” 7 and that Jacob had obeyed his father and mother and had gone to Paddan Aram. 8 Esau then realized how displeasing the Canaanite women were to his father Isaac; 9 so he went to Ishmael and married Mahalath, the sister of Nebaioth and daughter of Ishmael son of Abraham, in addition to the wives he already had. NIV, ©2011
Why did Esau only realize how much it bothered his parents then? They had evidently been quite angry about it for awhile, especially his mother.
Also, why the additional marriage? Why would that help, given that he was presumably still married to his Canaanite wives at that point?