Why is cell potential defined as E0[Reduction]-E0[Oxidation] and not the reverse?
It is universally accepted to express potentials as reduction potentials. However, one of the reactions is always going to be oxidation so you need the reverse. Normally cell potential is the SUM of reduction and oxidation potentials but the value you get for the oxidation reaction from tables is actually the reduction potential so you would have to change the sign. So you actually get:

Cell potential = (Reduction potential for the reduction at the cathode) - (Reduction potential for the oxidation at the anode)

or 

Cell potential = (Reduction potential for the reduction at the cathode) + (Oxidation potential for the oxidation at the anode)

because Reduction potential + Oxidation potential = 0 when the same species are involved