Moses waited until he was about to die before admonishing the Jewish people. One reason for this was that he wanted to wait until he had conquered the Amorite kings.
The Gift of Rebuke
אַחֲרֵי הַכֹּתוֹ אֵת סִיחֹן מֶלֶךְ הָאֱמֹרִי . . . וְאֵת עוֹג מֶלֶךְ הַבָּשָׁן וגו': (דברים א:ד)
[Moses rebuked the Jewish people] after he had smitten Sichon, king of the Amorites . . . and Og, king of Bashan. Deuteronomy 1:4

People accept rebuke more readily after having received some material benefit from the person giving the rebuke. By rebuking someone, we are doing them a spiritual favor, so by preceding this spiritual favor with a material favor, we ensure that both parties relate to the rebuke in the proper light – rather than considering it an act of ill will.

By his example, Moses showed us that this principle applies even when the individual or group is in need of rebuke for a sin as grave as that of making the Golden Calf. From Moses’ example, we learn that we should extend others our fullest help – both material and spiritual – in order to put them back on the proper path in life.

By helping others in this way, we earn G‑d’s help in finding our own proper path in life, as well as His assistance in providing for the material needs of ourselves and our loved ones.1