There are eleven species of hickory native to North America and yet the pecan is the only member of the Carya genus that has been developed into a commercially viable orchard crop. Ever wonder why? It's not all about taste. Sure some of the hickories produce bitter kernels, like the bitternut hickory (Carya cordiformis) and the water hickory (Carya aquatica). However, the shagbark hickory (Carya ovata) produces a sweet, oily kernel that has a delightful flavor.
This major difference in shell structure becomes obvious when cracking each nut. When a pecan is cracked the material inside the dorsal groove breaks free from the outer shell easing the extraction of kernel. When cracking a hickory, the shell's protrusions into the kernel remain firming attached to the outer shell and can trap portions of nut meat inside of shell fragments.
The yellow arrows, in the photo above, point to the inner wall partition of each nut. Note that this inner wall in the hickory is composed of the same hard material as the outer shell. In pecan, the inner wall is softer packing material similar to the material found inside the dorsal groove. When a pecan is cracked the inner wall breaks free from the outer shell allowing both kernel halves to fall free of the shell. In contrast, the inner wall partition inside a hickory nut is not easily separated from the outer shell wall during the cracking process. Often, when cracking hickory nuts, the inner wall will trap one of the two kernel halves requiring a second crack to fully remove all the kernel.