Because it was released before it was ready for the general public. It's too buggy. I think I'll wait.
https://www.youtube.com/watch?v=Ao7GJASlDOM
This guy is no authority. He thinks the "ding" noise for changing volume is gone. He thinks iWork 09 doesn't work. He thinks that changing to "dark mode" is for people who have vision difficulties.
I wouldn't put a whole lot of stock into anything he says. I'm not the world's biggest fan of Yosemite, but he seems downright ignorant.