with the Sechina & hashem in his holy presence & the great himble leader Moshe Rabeinu being with klal yisroel 24/7 it was a lack of trust & faith in Hashem to question Hashem about the land promised to them & not trust Hashem when they were on such a high level & saw such great miracles every day since they left mitzrayim over & over again.
i.e. its a insult & contradiction to question YOUR own father/parents raising you since birth & taking care of EVERY SINGLE NEED, when you never yet saw in your life them not taking care of you 24/7