Because the English translation causes you to think something different than what the Greek readers think when they hear it.
It is "Do not keep on clinging to me"
That is why the other English Translations have used words closer to the original Greek meaning.
New International Version
Jesus said, “Do not hold on to me, for I have not yet ascended to the Father. Go instead to my brothers and tell them, ‘I am ascending to my Father and your Father, to my God and your God.’”
New Living Translation
“Don’t cling to me,” Jesus said, “for I haven’t yet ascended to the Father. But go find my brothers and tell them, ‘I am ascending to my Father and your Father, to my God and your God.’”
English Standard Version
Jesus said to her, “Do not cling to me, for I have not yet ascended to the Father; but go to my brothers and say to them, ‘I am ascending to my Father and your Father, to my God and your God.’”
Berean Study Bible
“Do not cling to Me,” Jesus said, “for I have not yet ascended to the Father. But go and tell My brothers, ‘I am ascending to My Father and your Father, to My God and your God.’ ”
New King James Version
Jesus said to her, “Do not cling to Me, for I have not yet ascended to My Father; but go to My brethren and say to them, ‘I am ascending to My Father and your Father, and
to My God and your God.’ ”
New American Standard Bible
Jesus said to her, “Stop clinging to Me, for I have not yet ascended to the Father; but go to My brothers and say to them, ‘I am ascending to My Father and your Father, and My God and your God.’”
NASB 1995
Jesus said to her, “Stop clinging to Me, for I have not yet ascended to the Father; but go to My brethren and say to them, ‘I ascend to My Father and your Father, and My God and your God.’”
NASB 1977
Jesus said to her, “Stop clinging to Me, for I have not yet ascended to the Father; but go to My brethren, and say to them, ‘I ascend to My Father and your Father, and My God and your God.’”
Amplified Bible
Jesus said to her, “Do not hold Me, for I have not yet ascended to the Father; but go to My brothers and tell them, ‘I am ascending to My Father and your Father, and to My God and your God.’”
Christian Standard Bible
“Don’t cling to me,” Jesus told her, “since I have not yet ascended to the Father. But go to my brothers and tell them that I am ascending to my Father and your Father, to my God and your God.”
Holman Christian Standard Bible
“Don’t cling to Me,” Jesus told her, “for I have not yet ascended to the Father. But go to My brothers and tell them that I am ascending to My Father and your Father—to My God and your God.”
Aramaic Bible in Plain English
Yeshua said to her. “Don't cling to me, for I have not yet ascended to join my Father, and go join my brothers and say to them that I ascend to join my Father and your Father, my God, and your God.”
Contemporary English Version
Jesus told her, "Don't hold on to me! I have not yet gone to the Father. But tell my disciples I am going to the one who is my Father and my God, as well as your Father and your God."
Good News Translation
"Do not hold on to me," Jesus told her, "because I have not yet gone back up to the Father. But go to my brothers and tell them that I am returning to him who is my Father and their Father, my God and their God."
International Standard Version
Jesus told her, "Don't hold on to me, because I haven't yet ascended to the Father. But go to my brothers and tell them, 'I'm ascending to my Father and your Father, to my God and your God.'"
New American Bible
Jesus said to her, “Stop holding on to me, for I have not yet ascended to the Father. But go to my brothers and tell them, ‘I am going to my Father and your Father, to my God and your God.’ ”
NET Bible
Jesus replied, "Do not touch me, for I have not yet ascended to my Father. Go to my brothers and tell them, 'I am ascending to my Father and your Father, to my God and your God.'"
New Revised Standard Version
Jesus said to her, “Do not hold on to me, because I have not yet ascended to the Father. But go to my brothers and say to them, ‘I am ascending to my Father and your Father, to my God and your God.’”
Weymouth New Testament
"Do not cling to me," said Jesus, "for I have not yet ascended to the Father. But take this message to my brethren: 'I am ascending to my Father and your Father, to my God and your God.'"
World English Bible
Jesus said to her, "Don't hold me, for I haven't yet ascended to my Father; but go to my brothers, and tell them, 'I am ascending to my Father and your Father, to my God and your God.'"
Jesus was telling her to stop holding on to him as if she was afraid to let go and lose Him again. He was saying that when He goes to the Father He would send the Holy Spirit and He and the Father would take up their abode in Mary and that would be a better and closer relationship than what she was trying to hang on to with Jesus.
We read Him teaching throughout the book of John how he needed to go to the Father to send the Holy Spirit for this reason.
Now He is reminding Mary of what He had been saying. By this time in the book of John anyone who had read through the whole book knows exactly what "I must ascend to the Father" means. The Promise of the Holy Spirit.
Also since we see two Marys in Matthew actually embracing his feet and making contact and clinging to his feet we know that they did touch him after his resurrection before the meeting in Galilee. One left quit clinging to do what he said to go tell his disciples in Galilee while the other kept clinging to him.
@Magenta already posted both passages from Matt and John that show this.
The idea that Jesus did not let anyone touch him before Thomas is wrong. Matt 28
8And they departed quickly from the sepulchre with fear and great joy; and did run to bring his disciples word.
9And as they went to tell his disciples, behold, Jesus met them, saying, All hail.
And they came and held him by the feet, and worshipped him.
10Then said Jesus unto them, Be not afraid: go tell my brethren that they go into Galilee, and there shall they see me.
They did touch him. But in John what we see is that while one of them quit holding on to him and started to go obey what he said to do, (tell the disciples to meet him in Galilee) Jesus had to speak to the other Mary to tell her to quit clinging to him and remind her that He had to go to the Father and send the promise of the Holy Spirit which would then live in her all the time and she would never be separated from Jesus again, as He had promised to come and He and the Father would make their abode within her but he must first ascend to the Father and then He would send the Holy Spirit, which he did on the day of Pentecost. One ascension occurred later when they saw him rise.
The whole two ascension idea is based on misunderstanding the English translation "Do not touch me"