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.

SiC is a wide band gap semi conductor so it takes a much higher temperature for the intrinsic carrier concentration to rise to the point where it is no longer a semi conductor and just a conductor - it was 12 years ago since I finished my PhD in SiC and related devices so I can’t explain in much more detail but that’s the crux of it

There are many cases where mixed solid materials are stronger than the sum of their parts. Heat tolerance is all about high bond strength between the materials, and strategic mixing of materials can enhance that property or others

Chips aren't a single piece of Si or SiC crystal, but a superbly complicated arrangement of doped regions, insulating, metal and plating layers, so the melting point of the base crystal doesn't matter here (Si melts at ~1500 °C, C depends, and SiC seems to sublime at very high temperatures like carbon, instead of melting - corrections welcome). The limit to sustained high-temperature operation of ICs is due to all of these things starting to diffuse around and getting into places they're not supposed to be.

Keep in mind that SiC is a substance, so a pure SiC crystal is not a mixed solid.

Mixed solids are more sensitive to temperature variations (there's no change on the melting point), but actual chips are already full of mixed materials, and temperature variation isn't the bottleneck. A SiC chip has actually a bit fewer kinds of materials on the mix, but still, probably not enough to make a difference. (Anyway, we are talking about discrete transistors here, so the material count is exactly the same.)

You are probably thinking about pure substance (the ones that only have a single kind of chemical element). If so, you are completely wrong, pure substances usually have a very low melting point, and very few reach as high temperatures as you can get with composed ones.

It does have a lower melting point than diamond. The bond strength is determined by the electro-negativity of the bond pair. In this case, it is between silicon and carbon.

Crystals are conceptually very different from "mixed solids". Eutectics, e.g. solder, count as mixed solids.