People like to think that because diamond tops the Mohs scale of hardness it is some sort of ultimate super material. But that is far from the truth, diamond may be impossible to scratch, but it is also brittle and blunt trauma can shatter it.
So your suit of armor would be great if somebody attacks you with a drill or with a hacksaw, but god help you somebody actually swings a sword at you, because the force of a decent swing will likely turn your million dollar armor into shards.
I guess you could cover steel with diamonds instead, but the benefit would be minimal. Because steel is already pretty good at deflecting cuts. And you don’t plan to get attacked by people wielding industrial grade drills anytime soon right?