River gave him a cape by the end of Battle for Mewni and told him it was a knight one as payment for his aid in the conflict.
Marco grew insanely obsessed with the k ight title, which cost him his friends and girlfriend. In the end, he drops out of school just to live in Mewni as a knight.
But turns out that wasn't a knight cape but a meat blanket, because Moon and River wanted to get rid of Marco as they considered him a phase. So when he arrives, Moon, River and the royal knights laugh at him for mistaking a meat blanket and expecting a human to be knighted.
Eclipsa, on the other hand, respected everybody's right to be a knight as long as they showed what is needed for the job.
Her royal knights combine mewmans, monsters and humans (Marco), they are extremely loyal and trust each other.