Q. I went with my friend to her mom's funeral at a Protestant church where the pastor was a woman. I was wondering why the Catholic church doesn't allow women deacons or priests, but we do have nuns and other religions don't?
A. Good question. The answer is best understood when we look at the Scripture.
All of the Sacraments were, in some way, instituted by Jesus Himself in the Gospels. That means they were His idea, came from Him and are to be done in the way He gave them to us. For example, when baptizing someone we use water because that what was used in Jesus' baptism. And when we baptize, we do so "In the name of the Father and of the Son and of the Holy Spirit" because that's what Jesus told us to do when He gave us that Sacrament as he was preparing to ascend to Heaven. Similarly, we use wine and bread for the Holy Eucharist. Why? Because that's what Jesus used at the Last Supper. So we do what Jesus did.
When it comes to the Sacrament of Ordination, to the priesthood and deaconate, we also do what Jesus did. What did He do? Well, He gathered 12 men around Him as His apostles, prepared them for 3 years every day, and then, after His resurrection, He appeared to them in an upper room and "breathed on them and said, 'receive the Holy Spirit'" (John 20:20). For some reason He only picked men for this. Why? I guess we'll ultimately have to ask Him this question in Heaven. We can speculate and come up with reasons, but the bottom line is that we do what He did. Therefore, the Church has declared that we have no authority to do anything else. As a matter of fact, to change what he did would be like saying we can use grape juice for baptism or pizza for the Holy Eucharist. We simply cannot change what He gave us.
Perhaps one of the reasons He only picked men as apostles is because He became a man Himself. But, again, we'll have to ask Him that in Heaven.
One important thing to point out is that this, in no way, should be seen as discrimination against women. Nor should it ever be seen to imply that we think men are better. Our Church clearly believes that men and women are 100% equal in dignity. But we are different and are given different roles in life. Humility will help us all to see and appreciate the unique and different roles God has given to both men and women.
Deacons were first seen in the Acts of the Apostles and were instituted by Jesus through the guidance of the Holy Spirit. The first ones to be ordained deacons were 7 men. Again, this is something we cannot change.
But you are correct, we do have women nuns who are called to give their lives to God through certain vows and are called to live a very unique and sacred vocation within the church. Actually, all people are called to a unique and sacred vocation within the Church. We all have our callings and must answer that calling in the way God leads. When we do this, the Church becomes a beautiful reflection of God Himself.
As for the last part of your question, why the Catholic Church has nuns and others Protestant Churches do not, it's in part a historical question. As early as the New Testament times certain widows were acknowledged as having a special role in the community. Slowly over time this role became more organized. In the centuries to come, as the monastic life of men developed, so also the communities of widows and other consecrated women developed. The Catholic Church is blessed with almost 2,000 years of this development and transformation into the current structures of the religious life for men and women. Though religious life for men and women as a brother, monk or sister is not a sacrament, it is a special calling. For a more detailed look at the history of this see the link below.
Hope that helps!