Because it can be demonstrated that the story was fabricated from passages in the Septuagint, and Matthew and Mark both make mistakes which clearly show it.
Mark’s version took his story from Zechariah 9:9:
Rejoice greatly, O daughter of Zion! Shout aloud, O daughter of Jerusalem! Lo, your king comes to you; triumphant and victorious is he, humble and riding on an ass, on a colt the foal of an ass.
Now here is Mark’s version:
Mark 11:1-11
As they approached Jerusalem and came to Bethphage and Bethany at the Mount of Olive, Jesus sent two of his disciples, saying to them, “Go to the village ahead of you, and just as you enter it, you will find a colt there which no one has ever ridden… Untie it and bring it here. If anyone asks you, ‘Why are you doing this?’ tell him, ‘The lord needs it and will send it back shortly.’” They went and found a colt outside in the street, tied at a doorway. As they untied it, some people standing there asked, “What are you doing, untying the colt?” They answered that Jesus had told them to, and the people let them go. When they brought the colt to Jesus and threw their cloaks on it, he sat on it. Many people spread their cloaks on the road, while others spread branches they had out in the fields. Those who went ahead and those who followed shouted, “Hosanna! Blessed is he who comes in the name of the Lord! Blessed is the coming of the kingdom of our father David! Hosanna in the highest!” Jesus entered Jerusalem and went to the temple. He looked around at everything, but since it was already late he went out to Bethany with the twelve.
I’ve redded three mistakes here. First, Mark gets the order of Bethpage and Bethany wrong as they are approached from Jericho. A small thing, you might say. Agreed, but it hurts Mark’s credibility right off the bat. The next two mistakes are more significant, though.
In the second line I colored. Jesus tells the disciples they will find a colt that no one has ever ridden. This shows that Mark was a ware of a phrase which occurs in the Greek LXX translation of Zechariah, but not in the Hebrew. That phrase is polon neon - “new” (i.e. “unridden”) colt. Mark has Jesus do something to fulfill the Greek translation of the Bible that Mark had, but not a version that Jesus would have been familiar with or chosen over the Hebrew.
The third part I colored is very familiar, but I don’t think most people know that it’s a lingistic mistake by Mark. He takes that passage almost word for word from Psalms 118:25-26:
Hosanna, O Lord…
Blessed be he who enters in the name of the Lord.
Praise him in the highest!
Mark renders it thusly:
Hosanna!
Blessed is he who comes in the name of the Lord!
Hosanna in the highest!
Mark accidentally says “Hosanna in the highest,” at the end instead of “praise him in the highest,” which is what the Psalm says. The problem with this is that the word “Hosanna” in Hebrew means “Save us,” “help” “I pray.” It’s a cry for help, not a word of praise. “Hosanna in the highest” would be gibberish in either Hebrew, Aramaic or Greek, and no crowd in Jerusalem would have said it.
Matthew made an even dumber mistake. He (ridiculously) misunderstood Zechariah to be saying that Jesus rode on two animals at once (…riding on an ass, on a colt the foal of an ass…), and crafted his narrative accordingly:
Mt 21:1-7
As they approached Jerusalem…Jesus sent two disciples, saying to them, “Go to the village ahead of you, and at once you will find a donkey tied there, with her colt by her. Untie them and bring them to me. If anyone says anything to you, tell him that the Lord needs them, and he will send them right away.” This took place to fulfil what was spoken through the prophet:
Say to the daughter of Zion, “See your king comes to you, gentle and riding on a donkey, on a colt, the foal of a donkey.”
The disciples went and did as Jesus had instructed them. They brought the donkey and the colt, placed their cloaks on them, and Jesus sat on them
That pretty much cinches it. Matthew and Mark are both working from Zechariah, not history.