For me, it is about an inconsistency in the memory of the Gaunts.

Marvolo was clearly incensed by the fact that a muggle, Tom Riddle Sr and his parents, were richer then them and lived in the Manor House up on the hill. If he could have, he would have stollen the muggles money and house.

So, I think that Marvolo and his children were cursed. I think they were cursed to never gain money by illicit means.

It would explain why so many years later, Tom Riddle is basically a finacial parasite on his followers, he cant steal the money for himself.