Crawling for Time

The first time i saw the hard roll from the f m s that gray cook demonstrated u, well, that was thing that was supposed to be hard. And for me, it was almost impossible. Couldn't do it. So right from the getgo, could not crawl for any appreciable length of time. But then one day at the fire department, i was out back crawling in the rocks and on the back driveway a, i hit 12 minutes. Oside mentisis, why is it so hard?