Right after you wake up from a dream, you seem to be able to remember most if not all of the details. Then, over time, these details fade and often disappear.
Is this a form of regression? Why is it harder to solidify memories of dreams?
My suggestion would be that, as with all forms of mental imagery, short-term working memory circuits are responsible for keeping dreamt images active, i.e. in consciousness. Distraction by competing perceptual representations would cause immediate disruption of one's dreamt images; without such distraction, imagery in short-term memory typically starts to rapidly fade after about 8 seconds, unless it's highly relevant to one's immediate situation on waking, and hence promptly acted on.