Ex 6:9 So Moses spoke thus to the children of Israel; but they did not heed Moses, because of anguish of spirit and cruel bondage.

They didn't believe the good news of deliverance. This is what happens when we are consumed by our troubles than on God's promises. When we are more overwhelmed by circumstances than on God's truth.

To be sure, what God told Abraham was passed around and retold over and over again:

 Ge 15:13 Then He said to Abram: "Know certainly that your descendants will be strangers in a land that is not theirs, and will serve them, and they will afflict them four hundred years.

14 "And also the nation whom they serve I will judge; afterward they shall come out with great possessions.

If they kept this in their hearts, if they heeded it, they would have heeded Moses.