because

Exodus 15:23, 24
And when they came to Marah, they could not drink the water there because it was bitter. (That is why it was named Marah.) . . .

Exodus 16:2, 3
And there in the desert they all grumbled against Moses and Aaron. . . .

Exodus 17:2, 3
So the people contended with Moses, "Give us water to drink." "Why do you contend with me?" Moses replied. "Why do you test the LORD?" . . .

Numbers 11:1
Soon the people began to complain about their hardship in the hearing of the LORD, and when He heard them, His anger was kindled, and fire from the LORD blazed among them and consumed the outskirts of the camp.

Numbers 14:1-5
Then the whole congregation lifted up their voices and cried out, and that night the people wept. . . .

Numbers 16:41
The next day the entire congregation of Israel grumbled against Moses and Aaron, saying, "You have killed the LORD's people!"

Psalm 106:7, 8
Our fathers in Egypt did not grasp Your wonders or remember Your abundant kindness; but they rebelled by the sea, there at the Red Sea. . . .

wherefore

Exodus 5:22
So Moses returned to the LORD and asked, "Lord, why have You brought trouble upon this people? Is this why You sent me?

Genesis 43:6
"Why did you bring this trouble upon me?" Israel asked. "Why did you tell the man you had another brother?"

Numbers 11:15
If this is how You are going to treat me, please kill me right now--if I have found favor in Your eyes--and let me not see my own wretchedness."