Touch me not

Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and [to] my God, and your God. John 20:17

Why did Jesus tell Mary Magdalene not to touch him when he would tell other disciples to touch him later? What did He mean when he said, "I am not yet ascended"?

I found the Matthew Henry's insights quite helpful:

[3.] The further instructions that Christ gave her (v. 17): "Touch me not, but go and carry the news to the disciples.’’

First, He diverts her from the expectation of familiar society and conversation with him at this time: Touch me not, for I am not yet ascended. Mary was so transported with the sight of her dear Master that she forgot herself, and that state of glory into which he was now entering, and was ready to express her joy by affectionate embraces of him, which Christ here forbids at this time. 1. Touch me not thus at all, for I am to ascend to heaven. He bade the disciples touch him, for the confirmation of their faith; he allowed the women to take hold of his feet, and worship him (Mt. 28:9); but Mary, supposing that he was risen, as Lazarus was, to live among them constantly, and converse with them freely as he had done, upon that presumption was about to take hold of his hand with her usual freedom. This mistake Christ rectified; she must believe him, and adore him, as exalted, but must not expect to be familiar with him as formerly. See 2 Co. 5:16. He forbids her to dote upon his bodily presence, to set her heart on this, or expect its continuance, and leads her to the spiritual converse and communion which she should have with him after he was ascended to his Father; for the greatest joy of his resurrection was that it was a step towards his ascension. Mary thought, now that her Master was risen, he would presently set up a temporal kingdom, such as they had long promised themselves. "No,’’ says Christ, "touch me not, with any such thought; think not to lay hold on me, so as to detain me here; for, though I am not yet ascended, go to my brethren, and tell them, I am to ascend.’’ As before his death, so now after his resurrection, he still harps upon this, that he was going away, was no more in the world; and therefore they must look higher than his bodily presence, and look further than the present state of things. 2. "Touch me not, do not stay to touch me now, stay not now to make any further enquiries, or give any further expressions of joy, for I am not yet ascended, I shall not depart immediately, it may as well be done another time; the best service thou canst do now is to carry the tidings to the disciples; lose no time therefore, but go away with all speed.’’ Note, Public service ought to be preferred before private satisfaction. It is more blessed to give than to receive. Jacob must let an angel go, when the day breaks, and it is time for him to look after his family. Mary must not stay to talk with her Master, but must carry his message; for it is a day of good tidings, which she must not engross the comfort of, but hand it to others. See that story, 2 Ki. 7:9.