> I cannot understand why silicone rubber is not more commonly available for cable insulation

Silicone is more flexible and high-temperature resistant, but it does have a major downside of being very easy to cut and strip. That's not great either, especially for power applications where short circuits are a concern.

Silicone also kinda sucks for crimping. Most crimps have one set of tabs which grab the conductor (which is fine) and then another set which grab the insulation, but in the case of silicone the insulation set tend to just slice the insulation and don't mechanically grab it.