I wish I knew that!
Soot is close to graphite, and the closer a material gets to graphite, the more difficult it is to ignite. Graphite blocks serve as a liner in high temperature ovens because they resist in air at very high temperature. But they must be graphitized for long at high temperature to remove any hydrocarbon.
You can try with lubricant graphite. It's a powder down to 30nm grain size. Such a fine powder of iron, aluminium... would catch fire explosively by mere contact with air at room temperature. But Graphite powder can fall through a candle flame and it lands unburned. I'm sure the tiny grains get really hot in the flame.
Once soot is produced somewhere in a flame, it's very difficult to complete the combustion elsewhere, despite soot is initially very fine and hot, and it receives new oxygen. That's a hard limit to super fuels with highly strained cage molecules that contain too little hydrogen.
Vague hypothesis: the vapour pressure of graphite is far too small to feed a flame even when hot, and the solid does not burn because no reaction path can rip carbon atoms from graphite.