CO is a stronger ligand than NH₃ for many metals why?

According to Ligand field theory ; the  π-acceptor ligands are the stronger ligand and the π-donating ligand is a weak field ligand. here, we know that NH₃ is a  π-donor in comparison to the CO which is a  π-acceptor ligand

 In case of CO ;

 It  not only contain lone pair of electrons but also have vacant π or π^* -orbitals.  So, it works as π-acceptor and it forms a σ-bond with metal atoms by donating the lone electron pair and at the same time it also forms a π-bond  by accepting electrons from the filled metal orbitals. 
Hence, when CO approaches to an octahedral field, it has an unoccupied π-orbitals at energies greater than the metal d_xy ,d_xz , and d_yz orbitals .

So the mixing of both occurs to form π and π^* orbitals, where the  π^* orbitals are localized on ligands and not on the metal so the nonbonding d-orbitals before reaction, now become  π orbitals which increases crystal field stabilizing energy Δ_o .

Now, if Δ_o is gighers so the CO will be called as strong field ligand while in case of π-donor ligand like NH₃ , the process is reversed and the value of Δ_o becomes lower so they behave as weak ligand, because the π -orbitals are occupied in this case.

For diagrammatic representation: