In case of alkaline earth metals, the oxidation state more than two is not observed because

The alkaline earth metals in their valency shell contain only "2" electron hence they lose two electrons easiliy and shows +II oxdiation state by forming stable M+2  ion.But they can't show oxidation state more than twoReasonM+2 ion attain stable noble gas electronic configuration and hence the removal of 3rd electron is highly difficult , and the energy required for this purpose is extremely high .