Because of the way steemit works. The person with the most SP has the most influence, and earns new Steam Power more quickly.

At a certain point the whales who saved all of their SP, will have a really big influence. As the price of steem grows. It become's increasingly more diffficult to 'buy in'.

So as the platform matures, the newbies will have increasingly difficulty in earning their own SP. Making it very hard for them to stay competitive.