The reality of their situation was starting to bite: there would be no promised land for this group of Israelites. Their wish, to die in the wilderness, would be granted.
There was no hope of a better life: the dust, the hard work, and the inconvenience would be theirs until they died. Egypt looked better and better all the time:
Numbers 16:13-14 Isn’t it enough that you brought us out of Egypt, a land flowing with milk and honey, to kill us here in this wilderness, and that you now treat us like your subjects? What’s more, you haven’t brought us into another land flowing with milk and honey. You haven’t given us a new homeland with fields and vineyards.
They’d forgotten the oppression of Egypt, the miracles of their survival and God’s ongoing presence in the camp. They’d also forgotten that it was their choice not to go into a new promised land.
More likely, they’d realised exactly what they’d chosen, and were now trying to throw off Moses’ leadership in the hope they’d be able to find their own place to settle without the inconvenience of a holy God who wanted the best for them.