The reason Moses, Elijah, and Enoch were translated beings, who never suffered death as normal men do, is because their calling required them to remained in their bodies. Certain ordinances and blessings can only be given to others while in a body of flesh and bones. Thus they were blessed to remain in their bodies until they fulfilled their callings. These callings could have been fulfilled were they given the opportunity to receive a resurrected body, but, they could not receive a resurrected body since Christ was to be the first to resurrect.
Acts 26:23
23 That Christ should suffer, and that he should be the first that should rise from the dead, and should shew light unto the people, and to the Gentiles.
Colossians 1:18
18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.
Revelation 1:5
5 And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
Once Christ resurrected, there was no need for them to remain in a state of translation but could then receive the resurrection which was permanent. Those who are translated tend to be changed quickly and suffer no death in the grave but are changed in the twinkling of an eye.
There are those who were alive after Christ's resurrection who chose to remain on the earth and not die until Christ's second coming. John the Apostle and Revelator was one such being as also were the three Nephites of the Book of Mormon. But their fate is to never taste of death. Thus they cannot be the two witnesses either.
3 Nephi 28:6-10
6 And he said unto them: Behold, I know your thoughts, and ye have desired the thing which John, my beloved, who was with me in my ministry, before that I was lifted up by the Jews, desired of me.
7 Therefore, more blessed are ye, for ye shall never taste of death; but ye shall live to behold all the doings of the Father unto the children of men, even until all things shall be fulfilled according to the will of the Father, when I shall come in my glory with the powers of heaven.
8 And ye shall never endure the pains of death; but when I shall come in my glory ye shall be changed in the twinkling of an eye from mortality to immortality; and then shall ye be blessed in the kingdom of my Father.
9 And again, ye shall not have pain while ye shall dwell in the flesh, neither sorrow save it be for the sins of the world; and all this will I do because of the thing which ye have desired of me, for ye have desired that ye might bring the souls of men unto me, while the world shall stand.
10 And for this cause ye shall have fulness of joy; and ye shall sit down in the kingdom of my Father; yea, your joy shall be full, even as the Father hath given me fulness of joy; and ye shall be even as I am, and I am even as the Father; and the Father and I are one;
John the Beloved and the 3 Nephites are translated beings who are still alive on the earth today and when Christ shall make his second appearance on this earth, they will be changed from their translated state to a resurrected state all in the twinkling of an eye and shall not taste of death. This is the blessing upon those who have been translated and they will not suffer death as the two witnesses who shall be in Jerusalem and will suffer death and lay in the streets of Jerusalem for 3.5 days before being resurrected and caught up in the clouds of heaven.