They have invested with their work and sharing their knowledge. They are powering down and cashing out, because many people is willing to "buy" steem to invest in Steem Power.

I will try to answer to your question: interesting contents are increasing on Steemit, and every person that uses some social network has an advantage in starting using Steemit. So why no new user should come in? the only explaination could be that Steemit has 1 billion users. In that case we will see a world very different from the world we are living in, because our society is influenced in terrific why by the money creation based on debt!

I explored this topic deeper in this post:
https://steemit.com/steemit/@freiheit/is-steemit-our-occasion-to-save-the-world-of-course-and-we-ll-have-to-enrich-ourselves-while-we-do

Sorry if my engligh may have a couple of mistakes, I hope to improve right here :)