The Lord is a God who avenges.
O God who avenges, shine forth.
2 Rise up, Judge of the earth;
pay back to the proud what they deserve.
3 How long, Lord, will the wicked,
how long will the wicked be jubilant?
4 They pour out arrogant words;
all the evildoers are full of boasting.
5 They crush your people, Lord;
they oppress your inheritance.
6 They slay the widow and the foreigner;
they murder the fatherless.
7 They say, “The Lord does not see;
the God of Jacob takes no notice.”
8 Take notice, you senseless ones among the people;
you fools, when will you become wise?
9 Does he who fashioned the ear not hear?
Does he who formed the eye not see?
10 Does he who disciplines nations not punish?
Does he who teaches mankind lack knowledge?
11 The Lord knows all human plans;
he knows that they are futile. Psalm 94:1-11
Lillie-Put 
I always ask myself this question and cannot come up with a suitable answer. I believe it is due to unbelief and the hardness of their heat.
I am with you. The answer is beyond my understanding. Maybe there is more than one. I think pride figures in as well.