The difference is that Marco's only connected to the main story through Star.
His time with Hekapoo is barely brought up, his monster arm is forgotten in like 2 episodes. He fucking used the wand and it's never mentioned again.
Compared to Connie who is more of an equal to Steven in terms of story hierarchy. The only problem with Connie is that her role is unclear. She's the human anchor for Steven, but also part of Stevonnie, and also has her own sword and has training. The result is that we have a character who feels a bit out of place in all of these contexts.