Being D is not a portrayal of strength. There are weak Ds like Saul, Rouge, and Laws parents. On the other hand there are no weak conquerers. In fact every top tier/legendary pirate has CoC (not all of them have Ds though). I wonder why Oda would give this power that is basically a requirement to be a legendary figure to Luffy and Kidd but not Law. Coincidentally Luffy and Kidd are the ones with the rivalry relationship too....