A lot of the comments are crediting the immigrant parents as being "motivated" parents...maybe, but in this modern economy I think there is a more economic reason:
US born children have parents that more likely than not are indebted; whereas, even poor immigrants are unlikely to be indebted. It likely follows children whose parents not in debt, no matter how poor, likely have higher rates of upward mobility than children of indebted parents. There are very real glass ceilings in the US (having the lowest socio-economic mobility of all 1st world countries), so you are likely to raise from below the poverty line to just above, but if you are already above their isn't much likelihood of going up further.
US born children have parents that more likely than not are indebted; whereas, even poor immigrants are unlikely to be indebted. It likely follows children whose parents not in debt, no matter how poor, likely have higher rates of upward mobility than children of indebted parents. There are very real glass ceilings in the US (having the lowest socio-economic mobility of all 1st world countries), so you are likely to raise from below the poverty line to just above, but if you are already above their isn't much likelihood of going up further.