Wine had to be fully fermented to be kosher for Passover. They drank wine in Matthew 26. In Matthew 23, Jesus had said the scribes and Pharisees sat in Moses seat and to do what they ‘bid you observe.’

The Pharisees had them drinking wine that was completely fermented so all the yeast in the wine would die and they could slough it off the top. Grape juice could have yeast in it that hadn’t all died off yet, so it wasn’t kosher.