As for the actual question as to why Lucius Malfoy or one of the other fine upstanding Death Eaters didn't tell the Ministry, there was a good reason and a very good reason.
A Good Reason
Lucius always played things close to the vest. He knows knowledge is power, and just giving away secrets gets you nowhere. If he had told the Ministry, they would have tried to track down and arrest Sirius. And that's assuming one of Dumbledore's cronies in the Ministry, like Weasley, didn't pass the information on.
If they keep it to themselves, they can follow Sirius around. He might lead them to other Order members, or safe houses. Even if he can't lead them to #12, if they see him in the neighborhood enough, they will know to keep an eye on the Square. Or just firebomb it.
Of course this was all spoiled because Snape was a dirty double agent, but the Death Eaters didn't know that.
A Very Good Reason
It would be very difficult to pass the information to the Ministry without incriminating himself. Lucius obviously can't reveal that the information came from Wormtail, the supposedly dead man Dumbledore claims is a Death Eater and the reason Sirius was imprisoned. So what can he do?
Lie and say he has some sort of super Time Turner and went back to observe things? Unbelievable and illegal.
Claim he just remembered seeing Sirius transform when he was Imperiused to work for the Death Eaters? He looks stupid, incompetent, and suspicious.
Claim he just saw a dog on the street transform into Sirius (or verse vica)? He'll have to explain where and when, and why he let Black escape.
So yes, Malfoy knew. But no, he could not and would not tell the Ministry.