Why was Harv sick in Diamond Age? Wouldn't they have technology to cure those kind of diseases? In other parts of the book, they used nanocytes as defenses to to 'cure broken capillaries in the buttocks'. Besides, he seems to come from hardy stock.
The sickness comes from all the nano tech in the air.
As for why he wasn't cured, there's no specific cite but from the overall tone of the book Harv probably could have been cured if he was wealthy, powerful, connected, or "valuable". Since he's just a common street kid - tough luck.