It is because of the WT application of
James 5:14"Is anyone among you sick? Then he must call for the elders of the church and they are to pray over him, anointing him with oil in the name of the Lord; 15and the prayer offered in faith will restore the one who is sick, and the Lord will raise him up, and if he has committed sins, they will be forgiven him.…"
What they do is apply this as confessing only to the elders while they conveniently leave out verse 16:
"Therefore, confess your sins to one another, and pray for one another so that you may be healed. The effective prayer of a righteous man can accomplish much"
So confession to your spouse or another Christian is not acceptable enough to be forgiven by God even though Jesus died for our sins not them.