As they are covalent compounds
1 5 1
To conduct electricity a substance needs charged particles free to move. Metals have metallic bonding, so they have delocalised electrons free to move. Graphite, a covalent network compound, also has delocalised electrons. Ionic compounds conduct when molten or in solution because the ions break free from the crystal lattice and can move. Since carbon dioxide is covalent molecular bonding, it has neither delocalised electrons or ions, and this lack of charged particles free to move means it does
It is because in both tje compounfs center elements(C & Si) are from group 14(carbon family). group 14elements are covalent in nature and thus are good conductors of heat and electricity.