I know that way they would know Sirius black wasn’t guilty

They've made using Veritaserum in court cases illegal. This was done in part because Veritaserum isn't foolproof. People can take precautions to combat its effects, such as taking an antidote or using a spell to stop the potion from functioning properly. I think the reasoning for this is sound.

Imagine, if you will, the following scenario: someone who is believed to be under the influence of Veritaserum, loudly proclaims his innocence at trial. Everyone would accept that, because he shouldn't be able to lie. However, this person actually managed to use a transformation spell to turn the potion into water or a charm to prevent it from entering his body. He walks free, and immediately continues his crime spree. Yet he is never considered as a suspect again, because it wasn't him, right? Veritaserum proved that, right?

Suddenly relying on such means seems really tricky, wouldn't you agree?

@LectorMagnificus

I agree. Also, Veritaserum doesn't necessarily mean you'll even tell the truth; you tell what you think is the truth, even though you might be mistaken. This prohibition on using Veritaserum in court reminds me have how lie detector tests aren't admissible in courts. When you take a lie detector test, you might be deemed to be lying, when in reality, you're simply nervous. And a very good liar might be be able to beat the test. While it may often reveal whether someone's indeed being truthful or not, it still can be flawed. Veritaserum is quite the same way, I feel

That would've changed a lot of things! And it would have been much easier than hopping they would tell the truth