Why did Michelangelo present Mary in his Piet?

I think you are confused. The work is called The Pieta and it is Mary holding Jesus when he was taken down from the cross. It is beautiful and today is behind plexiglass in St. Peter's in Rome. When I saw it in the 1970's it was still in the original spot that Michelangelo placed it in St. Peter's. A man came along at one point and smashed it with a hammer so it was moved to safety.