His preference, I suppose, at least on this model. If you're going to do a saber grind with relatively thick stock like this, hollow is how you get the edge geometry down to realistically useable. And hollow doesn't have to mean thin--you can have a hollow grind that'll take chops into mild steel and a convex grind so thin it'll roll cutting cardboard.
On the big boy there I'd prefer a different grind as I consider knives of that size to be--at least partially--chopping/limbing tools. But in a 6" blade at 3/16" stock it's not going to be a chopping powerhouse no matter how you grind it.