Cause people think that it is a quick rich scheme they dropped out. The majors reasons are, according the me,:

• the poisened gift of a good introduction post. When writen well, it will get upvotes and some value, from there on they do think that every posts will have the same payout but most lickily the next 10 post will have a payout of around 0. Bye bye motivation.
• to make ik even worse they look at the hot and trending posts and don’t understand why they don’t get so many or big upvotes. Bye bye motivation!
  The ones which are staying do understand that hard work is required. If they are lucky somebody points out to them the force of genuine commenting. That were they start to create a healthy community and to get some SP!

On Steemit everybody has the same changes but you need to work hard to get them. In this aspect the Steem ecosystem is fair!