For the children of Israel walked forty years in the wilderness, until all the nation, even the men of war who came forth out of Egypt, were consumed, because they didn't listen to the voice of Yahweh: to whom Yahweh swore that he wouldn't let them see the land which Yahweh swore to their fathers that he would give us, a land flowing with milk and honey.
Why did God wait to command the circumcision of the new generation until now rather than on the plains of Moab? Perhaps He did so because He wanted to bring the people into the land before enforcing this aspect of the Law. This is consistent with God’s dealings with humankind. He first gives and then asks (cf. Romans 12:1).