Only Catholics can receive the Eucharist at Mass because the Eucharist is the Sacrament of our unity in Christ; those who receive it need to have unity in the Faith.
The practice of excluding some people from Holy Communion is a Biblically based, ancient Christian discipline observed by both Catholics and many Protestants.
Catholics believe that Jesus is truly present in the Eucharist, body, blood, soul, and divinity. If someone is not in communion with the Catholic Church, and does not believe what we do, the Church asks him to refrain from receiving the Eucharist with us, because the Eucharistic communion is a sign of unity in faith.
Those who reject Catholicism including Protestants and non-Christians, reject the doctrine of Transubstantiation. We Catholics believe that Jesus performs a miracle at every Mass and turns our offerings of simple bread and wine into His Own precious Body and Blood, but others do not believe this.
The Catholic Church isn’t doing something mean or intolerant. The Catholic Church is showing others how important the Eucharist truly is. For us, the Eucharist is the source and summit of our lives because It is Christ Jesus.
Many of those who do not agree, actually believe that our belief is morally wrong; therefore it makes no sense for them to participate in this sacrament.