As a musician I have constant ideas floating around in my head. Some of them are used, but the vast majority stay in there. I ask myself why are some ideas are more valid than others to be shared? I suppose there is no concrete answer to that. Is some art simply not good enough? If it is coming from the same talented artist, why would it not be good?
It's a strange thing to ponder but there are times when those old ideas come out like long lost friends and you realize how great they were to begin with. Sometimes they are even better than the ideas that came before them. Maybe our mind acts like a safe and keeps these things locked up in storage until the right time. The guess right time is usually when the artist decides they are ready to share.
Imagine peering into the mind of Davinci to see all the things he didn't share with us.