The Israelites murmured against Moses and Aaron in the wilderness because they doubted their leadership and, ultimately, God’s ability to provide for them.

In Exodus 16:2-3 (RSV):
"And the whole congregation of the people of Israel murmured against Moses and Aaron in the wilderness, and said to them, ‘Would that we had died by the hand of the Lord in the land of Egypt, when we sat by the fleshpots and ate bread to the full; for you have brought us out into this wilderness to kill this whole assembly with hunger.’”

This shows their lack of faith in Moses and Aaron as God’s chosen leaders and in God’s provision for them.