Been a while since I was in the chemistry game, but my guess is lattice structure and packing. Pure Si has relatively long bonds compared to carbon, so the inte

Been a while since I was in the chemistry game, but my guess is lattice structure and packing. Pure Si has relatively long bonds compared to carbon, so the intermolecular forces are a bit weaker. Apparently SiC has over 250 polymorphs, but the most common is the alpha. ... ok I'm reading about the material properties of SiC and it's insanely complex so I'm just going handwave and say that α-SiC has tighter packing with stronger bonds because of the alternating Si and C atoms.