Because the 2000 year old church that you are referring to fell into apostasy shortly after the deaths of many of the apostles.
The Great Apostasy
Following the death of Jesus Christ, wicked people persecuted and killed many Church members. Other Church members drifted from the principles taught by Jesus Christ and His Apostles. The Apostles were killed, and priesthood authority—including the keys to direct and receive revelation for the Church—was taken from the earth. Because the Church was no longer led by priesthood authority, error crept into Church teachings. Good people and much truth remained, but the gospel as established by Jesus Christ was lost. This period is called the Great Apostasy.
With the death of the Apostles of Jesus Christ, the truth was lost once again.
This apostasy resulted in the formation of many churches with conflicting teachings. During this time, many men and women sought the truth, but they were unable to find it. Many good people believed in God and Jesus Christ and tried to understand and teach truth, but they did not have the full gospel or priesthood authority. As a result, each generation inherited a state of apostasy as people were influenced by what previous generations passed on, including changes to Christ’s gospel.
God knew there would be an apostasy. Through an Old Testament prophet, He said:
“Behold, the days come … that I will send a famine in the land, not a famine of bread, nor a thirst for water, but of hearing the words of the Lord:
“And [people] shall wander from sea to sea, and from the north even to the east, they shall run to and fro to seek the word of the Lord, and shall not find it.”
Amos 8:11–12
Some inspired people, such as Martin Luther and John Calvin, recognized that practices and doctrines had been changed or lost. They tried to reform the churches to which they belonged. Without priesthood authority, however, Christ’s gospel could not be returned to its original form. A
restoration was needed.
Peter taught the following after Christ's resurrection and ascension into heaven:
Acts 3:20-21
20 And he shall send Jesus Christ, which before was preached unto you:
21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
If Jesus is again to be sent unto us during the times of the restitution of all things, then surely there would be a falling away and a need for a restitution of all things which would necessitate Jesus coming again to restore his gospel. I believe that it was through the Prophet Joseph Smith who is the head of the dispensation of the fulness of times, that Jesus came and restored his gospel again upon the earth.
Ephesians 1:10
10 That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:
Apostasy of the Early Christian Church
- changed the ordinance, broken the everlasting covenant, Isa. 24:5.
- this people draw near me with their mouth, Isa. 29:13.
- darkness shall cover the earth, Isa. 60:2.
- a famine … of hearing the words of the Lord, Amos 8:11.
- his enemy came and sowed tares among the wheat, Matt. 13:25.
- saying, I am Christ; and shall deceive many, Matt. 24:5.
- shall arise false Christs, and false prophets, Matt. 24:24.
- his disciples went back, and walked no more with him, John 6:66.
- shall grievous wolves enter in among you, Acts 20:29.
- there be divisions among you, 1 Cor. 11:18.
- I marvel that ye are so soon removed from him, Gal. 1:6.
- who hath bewitched you, that ye should not obey, Gal. 3:1.
- shall not come, except there come a falling away first, 2 Thes. 2:3.
- some having swerved have turned aside, 1 Tim. 1:6.
- giving heed to seducing spirits, 1 Tim. 4:1.
- all they which are in Asia be turned away from me, 2 Tim. 1:15.
- Who concerning the truth have erred, 2 Tim. 2:18.
- Having a form of godliness, but denying the power, 2 Tim. 3:5.
- turn away their ears from the truth … unto fables, 2 Tim. 4:4.
- profess that they know God; but in works they deny him, Titus 1:16.
- From whence come wars and fightings among you, James 4:1.
- false prophets also among the people, 2 Pet. 2:1.
- being led away with the error of the wicked, 2 Pet. 3:17.
- now are there many antichrists, 1 Jn. 2:18.
- many false prophets are gone out into the world, 1 Jn. 4:1.
- certain men crept in … denying the only Lord God, Jude 1:4.
- which say they are apostles, and are not, Rev. 2:2.
- thou art lukewarm, and neither cold nor hot, Rev. 3:16.
- to make war with the saints, Rev. 13:7.
- formation of that great and abominable church, 1 Ne. 13:26.
- the Gentiles … have stumbled, 2 Ne. 26:20.
- Gentiles … will be drunken with iniquity, 2 Ne. 27:1.
- transfigured the holy word of God, Morm. 8:33.
- strayed from mine ordinances, and have broken mine everlasting covenant, D&C 1:15.
- Satan … soweth the tares, D&C 86:3.
- darkness covereth the earth, and gross darkness the minds of the people, D&C 112:23.
- they were all wrong; … their hearts are far from me, JS—H 1:19.
- See also Matt. 24:11; Rom. 11:21; 1 Cor. 1:11; 3:3; Col. 2:22; 1 Tim. 1:19; Titus 1:10; 2 Pet. 2:22; 3 Jn. 1:9; Rev. 2:5; 1 Ne. 11:34.