Because a half-filled subshell is more stable than an almost half-filled subshell.
Although different people prefer to write their electron configurations differently, I prefer to organize the orbitals based on the order in which they are filled.
Instead of chromium having a
Half-filled subshells are more stable because of their symmetry. Based on Hund's rule of maximum multiplicity, electrons will fill each orbital with one electron before pairing electrons. A half-filled subshell will have every orbital filled with exactly one electron, making it more stable.