Answer
Salam Dear Reader,
Thank you for your question and for contacting Ask About Islam.
Briefly, we need to explain the situation that existed at the time of Jesus (peace be upon him). First, Jesus was sent to the People of Israel, among them were the Pharisees who were the most corrupt and were the ones who plotted to kill Jesus (peace be upon him).
In order to prove that he was not a Prophet of God, they wanted to kill him by crucifixion, since this was the most humiliating form of execution at that time.
We also need to understand that Jesus (peace be upon him) was a mighty Prophet of God. He raised the dead and cured the incurable deceases by God’s will. So for someone to possess such powers and be crucified is a direct blow on God’s power; the idea that the one who raises the dead, himself dies the most humiliating death.
God made someone else appear to be Jesus; supposedly the disciple who betrayed him and told the Romans of his whereabouts. Jesus was again seen by his disciples after his alleged crucifixion. Jesus’ disciples witnessed that he indeed was never crucified since they touched and felt him; the Christian doctrine adds that there were marks of nails, etc… However, there is nothing to prove such allegations.
In addition, Jesus’ disciples knew that he was raised to the heaven as Allah says in the Quran. This concept is also in the Christian doctrine, the Ascension.
After supposedly being dead, Jesus (peace be upon him) was seen again:
[And came out of the graves after his resurrection, and went into the holy city, and appeared unto many] (Matthew 27:53)
[Then the eleven disciples went away into Galilee, into a mountain where Jesus had appointed them. And when they saw him, they worshiped him: but some doubted] (Matthew 28:17)
It is worth mentioning that the worship that is mentioned in the above biblical verse is merry excitement for seeing Jesus again after allegedly being crucified; therefore, some doubting that it was him. After approximately forty days, Jesus ascended to the heavens and was seen by eleven of his disciples.
We all know that Jesus had twelve disciples, while only eleven saw him ascending to heaven; this confirms that there was a disciple who betrayed him, therefore he was crucified instead of Jesus. Some Christian historians claim that he committed suicide, but again there is no clear proof.
So the whole idea of a mighty Prophet of God such as Jesus (peace be upon him) to be crucified is most humiliating and a direct insult to God’s will and power. This was already the talk:
[Likewise also the chief priests mocking him, with the scribes and elders, said, He saved others; himself he cannot save. If he be the King of Israel, let him now come down from the cross, and we will believe him] (Matthew 27:41-42)
Now why did God want the whole situation to appear as if he was crucified? And the simple answer is to keep the evil people away from Jesus and to punish the one who deserves such punishment.
Also to test the faith of his disciples and those who followed him; were they willing to sacrifice and fight back? The answer to this is obvious, they were weak and did not defend Jesus, since they, like everyone else thought that it was Jesus who was being crucified; it was only later that they discovered that he was not. So the crucifixion of a betrayer who looked like Jesus was God’s justice and test.
Now let us assume that there was no betrayer or the disciples were ready to fight back, what would have happened? Jesus and his disciples would have joined forces to stop the evil people and the ones who rejected faith and opposed the message of Jesus. God would have surely given them victory, the same way he gave victory to David against Goliath and the way he gave victory to Muhammad against the Quraish:
{And when Saul set out with the army, he said: Lo! Allah will try you by (the ordeal of) a river. Whosoever therefore drinketh thereof he is not of me, and whosoever tasteth it not he is of me, save him who taketh (thereof) in the hollow of his hand. But they drank thereof, all save a few of them. And after he had crossed (the river), he and those who believed with him, they said: We have no power this day against Goliath and his hosts. But those who knew that they would meet Allah exclaimed:
How many a little company hath overcome a mighty host by Allah’s leave! Allah is with the steadfast.} (Al-Baqara, Chapter 2, Verse 249)
To summarize the situation, the followers of Jesus were unlike Prophet Muhammad’s followers, they were not willing to sacrifice so that God’s religion is established. Therefore, the mission of Jesus was cut short; he conveyed God’s message by delivering the Gospel but was not able to establish God’s religion, where he hoped that the People of Israel will follow him and fully support him. 
The Gospel I am talking about is the original and untouched version that was revealed to Jesus and delivered to the People of Israel in its original form and language. Jesus’ message as we all know was corrupted and God sent Muhammad (peace be upon him) to deliver the final and complete message.
When Jesus (peace be upon him) returns towards the end of time, as believed by both Christians and Muslims, he will fight and defeat evil; therefore, continuing his mission that he was not able to complete in the first place.
I hope this helps answer your question.
Salam and please keep in touch.