Why do the transition elements exhibits variable oxidation states?

In transition elements, the $d$-orbitals are incompletely occupied in such a way that there oxidation state differs by unity.

The general electronic configuration of transition elements is $(n-1) d^{1-10} n s^{1-2}$. Due to very less difference in the energies of ( $n-1$ ) $d$-orbitals and $n s$-orbitals, the transition elements exhibit more than one oxidation states as electrons of both these shells can participate in chemical bonding. The $4 s$-electrons are responsible for lower oxidation state while both $3 d$ and $4 s$ electrons are responsible for higher oxidation states in first transition series.