Materials that conduct electricity well, like metals, also tend to conduct heat. For instance, a metal spoon left in a hot cup of tea will get hot, while the ceramic mug remains cool. This is because good electrical conductors are usually good heat conductors too.
So why is the conductivity hardly lower than that of the substrate, which is simple SiO2 and thus a somewhat mediocre insulator? How is this special? 10 % better than glass is nothing.