For some reason, these people have decided dreams are worth remembering. Sometimes it is because they had one that seemed to come true, or one that fascinated them. But we also know it because some low recallers are stimulated to recall when they read about dreams or take a class on them. Some low recallers seem to be less good at tasks involving visual imagination, such as when they have to look at a picture of a building made of blocks and then construct one out of blocks that are sitting in front of them. As far as personality factors, which many people might think to be the main factor, studies using several personality tests don't show either large or consistent relationships.
Several studies are pretty convincing on that point.
In extreme cases of excessive dream recall, or no dream recall at all, there may be chemical imbalances at work. In extreme cases of no recall, still another thing may be going on. These people may be dreaming very little or not at all. But the people who did that research in the 1950s and early 1960s may have jumped the gun a little because they hadn't studied the full range of people. Based on recent studies, including one using low recallers who were very low on visuospatial skills, it now seems probable again that some people don't dream.