why did parents forbid the man from going out with the old ma?

Manolin's parents saw the old man no longer had luck fishing, so they made Manolin go out with another boat.

But after forty days without a fish the boy’s parents had told him that the old man was now definitely and finally salao, which is the worst form of unlucky, and the boy had gone at their orders in another boat which caught three good fish the first week.