Hmm maybe because kings haki manifests in those who has/had the "potential" (based on ambition, determination, charisma, tenacity) to become pirate king. Imo marines simply does not have this and lack the mentality for this because they are serving under an establishment that embodies no part being greater than the whole. A marine influence can only go so far while pirates (in theory) have unlimited influence, it's just how they use it. Conquering their ambitions to stand a top everyone.
Zoro and mihawk may display kings haki too even though they have zero intention of being pirate king. Their ambition and drive to reach their goal (the WGS title) can be viewed as being ambitions as going for the pirate king title, standing atop the world in their own right.
Marines will always be under the 5 elders and CC so in a sense their ambition to stand at the top is already suppressed from the start.