In the form of dichromate, Cr(VI) is a strong oxidizing agent in acidic medium but Mo(VI) in $Mo{O}_3$  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.