Because they believed not in God
That he was able to give them bread, and provide flesh for them, or bring them through the wilderness to Canaan's land, as he had promised. God, and he only, is the object of faith, and he is to be believed in at all times, and for all things temporal and spiritual; and nothing is more displeasing to him than unbelief; for as faith gives glory to him, unbelief reflects dishonour upon him; faith sets its seal to him as true, but unbelief makes him a liar; and what is more provoking to man than to have his veracity called in question, and to be counted a liar? in short, as faith has salvation annexed to it, unbelief has damnation, and to whom did the Lord swear that they should not enter into his rest but to them that believed not? so great an evil is unbelief, and is the sin which "easily beset" F3 the Israelites, as appears from the context; see ( Hebrews 3:12-18 )
and trusted not in his salvation;
which he promised them, and bid them stand still and see, ( Exodus 14:13 ) , and of which they had had some proofs and instances in leading them through the Red sea, and thus far guiding them through the wilderness, and providing for them; and therefore had reason and encouragement to trust in the Lord, that he would yet be with them, and save them, and complete the mercy promised unto them.
F3 (euperistaton) , which Suidas, in voce (enperis) interprets a foolish thing; and it is thought by his learned editor Kusterus, in ibid. to allude to foolish persons, who stand round about a mountebank or juggler, gazing at his tricks with pleasure and admiration, being insnared by them.