Firstly, pensive memories aren’t proof because they can be altered (eg slughorn in HBP)

Also it would be next to impossible to even get anyone to listen enough to look at the pensive.

Snape was unlikely to speak up for Sirius, Lupin wouldn’t be listened to because werewolves aren’t respected and the other three are underage. That leaves Dumbledore as the only respectable witness, but even he is just going off gut instinct, he doesn’t have any proof.

On top of that, Sirius had acted like a criminal: he had broken into Hogwarts and attacked the fat lady, and he went into Harry’s dorm with a knife. He had laughed when 12 people were murdered.

IF Dumbledore and Lupin came forward, it would also very strongly suggest that they helped Sirius escape when he was captured in PoA. That would put them in danger of being arrested with little chance of success in clearing Sirius’ name.