I was reading this article and am somewhat confused by this passage

First, I’m assuming to “break and avalanche closure” is to go into a blocked off area due to avalanche risks. The part I’m confused about is “skier just can’t keep up”. Why wouldn’t a skier be able to keep up? I don’t have a ton of big mountain experience only going every few years, but I can’t figure out why this would be the case.

For what it’s worth I ski and snowboard and have done both on big mountains.