There are a few reasons for this. It is where the phrase “to be in communion with” comes from. But most importantly it has to do with the recognition of the Real Presence - body, blood, soul, and divinity - of Christ in the Eucharist. Protestants believe communion is merely a symbol for Christ. As saint Paul said “For he that eateth and drinketh unworthily, eateth and drinketh judgment to himself, not discerning the body of the Lord.” (1 Cor 11:29)