Because there is a fixed # of rewards and all 14k posts per day compete for those rewards. The accounts with any steem power are self-voting themselves to make more $. The new users read the 1-year old posts when you could earn a few hundred or a thousand if you got lucky. That's impossible now. The trending posts you see are all large accounts voting themsevles and their followers trying to earn an upvote by voting along. When new users stop buying steem, the entire gambit collapses. The big accounts will start to power down soon.