Because their Dumb####s

This is explained in the books themselves. The serum only makes the person say what they believe to be the truth. Someone who misinterpreted what they saw, trauma corrupted their memory, or through a memory charm what they believe to be the truth can be changed. If they were then given the serum they would say what they think is the truth, but it would be a lie.

That's why its not used in courts.

Dumbledore probably has some control that prevents them from using those methods the ministry officials would've been in trouble with him if they subjected Harry to those methods even if they worked.

Veritaserum is not admissible in court, for the very reason @Rosie. Geee gave. It kind of reminds me of how polygraph/lie detector test results aren't admissible in court; those aren't always reliable, because someone who's truthful but nervous, for example, might fail, whereas someone could be lying, but knows how to beat the test (the difference between lie detectors and truth serum is, of course, that with the serum, someone is compelled to tell what they think is the truth, whereas, with lie detectors, no one is compelled to say anything in particular)

Fudge hated harry too, guys xd

^ That's true; he certainly wasn't wanting to make things any easier for Harry!

Yep, thus my answer lmao

I always thought that trial was more the ministry trying to further damage Harry’s credibility than anything else. Fudge was doing everything he could to convince the wizarding world all was well and Harry’s news was very inconvenient. During the trial when Dumbledore offered to call another witness to testify, Fudge argued back that it would stretch out the trial. Harry’s innocence or guilt was pretty irrelevant to Fudge as his primary objective was to paint Harry and those taking his side as not worth listening to, not to get to the truth.  

Harry Potter saved a life period

The trial was an excuse to ruin Harry publicly instead of getting real evidence. all he did was use a patronous charm on a dementor and they were subjecting him to humiliation. Fudge was just so corrupt he wouldn't listen to any testimony.