It's easy to look in hindsight knowing he is there and what happens. When it is unfolding, you can't tell that the rider way up the road is going to slow down significantly.
+1
With the number of riders that were present in that group and the overall speed, it would be next to impossible to spot
one guy slowing down and then have enough time to check and react.
As somebody said on the youtube vid, "even pros crash".