Carbon's valency is not written as 1s2 2s2 2p2 because it does not accurately represent the element's electronic configuration. Carbon has an atomic number of 6, which means it has six electrons. The correct electronic configuration for carbon is 1s2 2s2 2p2 1p1.

In the notation you provided, 1s2 2s2 2p2, all three valence shell orbitals (2s and three 2p orbitals) are completely filled with electrons. This configuration would result in a stable noble gas configuration, but it is not the case for carbon. Carbon has only four valence electrons (two in the 2s orbital and two in the 2p orbitals), and it needs to gain or lose electrons to achieve a stable configuration.

The correct notation, 1s2 2s2 2p2 1p1, indicates that carbon has two electrons in the 1s orbital, two electrons in the 2s orbital, two electrons in two of the 2p orbitals, and one electron in the remaining 2p orbital. This configuration represents carbon's excited state, where one of the 2p electrons is promoted to a higher energy level. This unpaired electron in the 2p orbital is responsible for carbon's chemical reactivity and its ability to form covalent bonds with other atoms.

Therefore, the electronic configuration of carbon is accurately represented as 1s2 2s2 2p2 1p1, showing its tetravalent nature and its tendency to form bonds to achieve a stable noble gas configuration.