With lesser moisture content steamed wood that will be bent, the species known for their high bendability, if you leave on the sap wood, that is where your crack will start, and why it is removed.
That is interesting, because when making "natural" ( as in non-laminated) bows out of yew or elm, you have the sapwood as the front side of the bow and heartwood in the back, since the former handles being stretched better without cracking.