I don't know if this has been answered before but

I've never understood that myself. I watched this the other day. If he could admit to Sue Ann that he tried to kill Dixie years earlier & all the other things he did, it seemed he would've admitted singing to her. Only thing I can think of is maybe he didn't remember.

I think that he said he didn't remember because he was afraid if she asked him to sing it he would of gotten too emotional, like he didn't want to go there