Some plausible premises that have been kicking around in my head for a while, and lately found each other:

• When we don’t have concepts for things, we can hardly think about them. When I learn new concepts, I often notice them applying everywhere where before I didn’t even notice anything missing. This is a partial explanation sometimes given for us having few memories from childhood; we don’t have the concepts to think explicitly about our experiences when we are very young, and without that it is hard to record them in memory.
• We don’t invent our own concepts very much; we mostly inherit them from our society. How many concepts that you have did you invent? If it is very few, this is probably not just because society has already found all the useful concepts and given them to you. If you lived a thousand years ago, my guess is that society wouldn’t have given you concepts like ‘subjective probability’ or ‘tragedy of the commons’ or ‘computation’. And no matter how nerdy you are, you probably wouldn’t have made them up. After all, a whole bunch of people did live then, and they didn’t make them up.
• It’s hard to share your feelings in much detail with other people. We can all learn to use the same word ‘angry’ when a person has marked external symptoms of anger. But it’s hard for an angry person to tell you anything very nuanced about how exactly they feel.
• I find it hard to remember what emotions and things like that feel like. e.g. right now I’m not really sure what anger feels like. I know what it would be in response to, and that it’s kind of  bad, but sometimes enjoyable. And that it might cause snide remarks or energetic ball kicking and that sort of thing.

Hypothesis: We have relatively few concepts for the world inside our heads, because it’s not very shared, and we get concepts mostly from other people. This means it is hard to think about the world inside our heads, and so also hard to remember. (This is all relative to the world outside our heads, and relative to how we would be if we could show one another inside our heads more).