This is because when Elly was done talking on the phone with Thelma's friend Ed, her body language and tone of voice would tell anyone with properly-functioning senses that something had gone very wrong with Thelma indeed. If I were to come across Elly talking in a hushed tone of voice and looking as if something terrible had happened, I would ask her what was wrong. John did not because her tone of voice and facial expression meant almost nothing to him. John assumed that he was about to be invited to give the bride away because he's not good at reading body language and he doesn't seem to understand what to say in a given situation. This is a man who doesn't understand that when he's talking to his wife's brother's girlfriend, he shouldn't tell her that he's probably got a thing for the woman she replaced so there is something wrong with Trash-Bag Johnny.
The reason this seems to matter is that his seeming inability to read body language or understand social nuances has blinded and will continue to blind him to the unhappiness of those around him. We began this toboggan ride with him ascribing Elly's clear hatred of the life she led to crazy woman hormones and we'll end with the stupid sack of shit making arch comments about princesses because he's too bloody stupid to understand why April feels alienated.