Why didn't Baba build a nicer house for Hassan and Ali?

Baba has very much regret in his life over Hassan. Although Baba always admired Hassan's courage he felt that he could never treat him like a son. Hassan was a Hazara (half breed) and hence relegated to the status of servitude. Baba all but abandons Hassan, favouring Amir in the material parts of life. This was Baba's way of appeasing his guilt over Hassan's very rough life.