Answer/Explanation:
Because blue eyes are a recessive gene? So like if both parents have brown eyes (Dominant gene) Then that gene sort of "overpowers" the blue gene? Also, no matter how many children they have it doesn't mean, that one of them might not get blue eyes if both parents carry blue eye genes. They also could just end up with only brown eyed children, it's almost like spinning a wheel of fortune tbh lol
I find this sort of stuff hard to explain but I hope what I said helps you out :)