Or HaChaim on Exodus 16:6/אור החיים על שמות ט״ז:ו׳

1 ערב וידעתם כי ה׳ הוציא אתכם, "in the evening you will realise that G'd took you out, etc." The fact that the Israelites had not previously asked for meat is proof that they had considered such a request unrealistic, that they did not believe that they would ever be supplied with meat in the desert. They did not even mention the word meat until they wished they had died in proximity to the flesh-pots, etc. G'd now was anxious to demonstrate that He Who had taken them out of Egypt and crushed their enemies could also provide for them wherever they were. G'd intended to teach the Israelites a moral lesson, such as we find in Isaiah 44,18 where the prophet scores the people for "having besmeared their eyes so that they neither saw nor understood." G'd insisted that they would realise that He had taken them out of Egypt when they observed what would happen in the evening as opposed to what would happen in the morning (seeing that they had considered the supply of meat the most unlikely thing that could happen).