Only because they were seen as the followers of a fake prophet. Had he really been risen, all he'd have to do is appear before that High Priest, and Pontias Pilate, and that would have been it. They would all be vindicated.
You ignore the Gospel record. Christ did many miracles and yet people still did not believe. Seeing isn’t believing; believing is seeing.
Both sides—Roman leaders and Jewish leaders—were severely threatened by Jesus and by his following. The only reason from their perspective that he was a threat was because his followers believed him—not because anyone thought he was a fake prophet.
If a body were available, they would have produced it. Keeping their power intact meant crushing any threat.
The Jewish leaders paid the Roman guards to spread the story that the disciples had stolen the body.
These leaders are a good example of when a truth is known factually but not followed morally. You know the song that says “do you believe in rock and roll”? He’s not talking about believing mentally in the existence of rock and roll but believing from your heart. The Jewish leaders and Pilate had plenty of evidence to know the truth of Christ. They simply didn’t want, in their heart, to follow him.
He wasn’t a false prophet...that’s for sure...in 70 ad Jerusalem was sacked and destroyed...as Jesus prophesied!
Something about New Testement accounts that you can't accept?
He was buried, and that He rose again the third day according to the Scriptures, and that He was seen by Cephas, then by the twelve. After that He was seen by over five hundred brethren at once, of whom the greater part remain to the present, but some have fallen asleep.
1 Cor 15.4-6