Why can I remember some dreams very vividly for a long time, and others I forget within a minute of waking up?

Dreams are more difficult to remember than regular life events because we have a weaker sense of self in our dreams. Because of the principle of encoding specificity that weaker sense of self will give you less to work with when you try to construct your memory. This explains why dreams in which you are more lucid, are easier to remember. The emotional salience of events in the dream are more connected to aspects of your personality. This gives you more triggers to locate the memory. Said another way, you're simply more you in the dream and things that happen to you are stored in ways that you know how to retrieve, not in the ways that some ephemeral semi-you likes to store things.