When people suffer, all they want to hear is that their suffering will be removed. They do not care to hear about success in the future. It is so far removed from their present reality that they cannot relate to it.

Therefore, YHVH told Moses to just tell them that they will be taken out of Egypt, without any mention of a bright future. This is an important principle when trying to give people support and encouragement. If we give them a picture that is beyond their present ability to relate to, our words will not be comforting even if we have very good intentions. Telling someone who is in deep emotional distress, "Don't worry, all will be well in the future" might not have a positive effect. Better to show him how to get out of his present pain and only then to give a more optimistic message.