Why do some transition metals have multiple oxidation states?

Trasition metals can have multiple oxidation states because of their electrons

if two electrons have the same n, the same ℓ, and the same ml, then they are in the same orbital. the pauli exclusion principle states that no two identical fermions (a fancy word for electrons and other subatomic particles like electrons) may occupy the same quantum state in an atom simultaneously.

a electronegativity decreases beacuse the size of nucleus is bigger.