Why Steemit started to reward Steem? 为什么Steemit平台开始奖励Steem了？

Hi all, I found that Steemit had astarted to reward Steem rather than Steem Dollar recently (I think most of the people already found out,perhaps?), may I know why is it so?

大家好，我发现了Steemit平台最近开始以Steem来奖励大家了（我想多数人都有留意到吧?)，有谁能够跟我说一下是为什么吗？