I propose a different explanation: The covenant of circumcision was originally a covenant between G-d and Abraham, whereby G-d promised to make Abraham: 1) "extremely fruitful" and 2) to give his children "the land where you are residing" (i.e, the Promised Land) (Gen 17:6-14). G-d also told Abraham that this covenant would be a permanent one between G-d and Abraham's progeny for the generations; As for Abraham's part, he was to circumcise every male in his household (Gen 6:12-13).
BUT in the wilderness, we see that, because of the people's lack of faith in G-d (Gen 14:11), they were not allowed to cross into the Promised Land (Number 14:23), and, I would argue, thereby nullified (at least temporarily) the Abrahamic covenant and with it the obligation circumcision (why require them to circumcise themselves if they were unwilling to receive its benefit?). Since the children of Israel were unable to go into the Promised Land, G-d did not impose circumcision upon them as circumcision alone was a sign of obedience and contrition - which the Children of Israel were definitely not displaying when they "despised" G-d (Number 14:23). It was only after the next generation rose up and trusted in him enough to be obedient and cross the Jordan into the land of promise (and giants), would G-d be able to fulfill his promises to Abraham of fruitfulness and place. Thus only then would the covenant be re-ratified, and only then would the requirement of circumcision be required.
This is just as an aside, but one might be able to argue that, given G-d's telling Moses that he would destroy these people and make from Moses a new nation many times (Exodus 31:10, Numbers 14:12, Numbers 16:21; Deuteronomy 9:14), perhaps G-d did not require circumcision because he just wasn't sure whether they were going to make it out of the wilderness alive, thereby nullifying his promise to Abraham of fruitfulness, and once again negating the need for circumcision.
