In the form of dichromate, Cr(VI) is a strong oxidizing agent in acidic medium but Mo(VI) in MoO3 and W(VI) in WO3 are not because
Cr(VI) is more stable than Mo(VI) and W(VI)
Mo (VI) and W (VI) are more stable than Cr(VI)
Higher oxidation states of heavier members of group-6 of transition series are more stable
Lower oxidation states of heavier members of group-6 of transition series are more stable.
In d-block the stability of higher oxidation increases are more from 3d to 4d and then to 5d