John Wesley on Numbers 11:21

Six hundred thousand footmen - Fit for war, besides women and children. That Moses speaks this as distrusting God's word is evident; and that Moses was not remarkably punished for this as he was afterward for the same sin, Num 20:12, may be imputed to the different circumstances of this and that sin: this was the first offence of the kind, and therefore more easily passed by; that was after warning and against more light and experience. This seems to have been spoken secretly: that openly before the people; and therefore it was fit to be openly and severely punished to prevent the contagion of that example.