When women admit their feelings, they expect an answer to the confession; yes or no? If it's a no, or an awkward avoidance of the whole topic which they take as a rejection, they'll try hard to get over it and move on, instead of lingering and pining embarassingly after someone who doesn't reciprocate their feelings.
It's what I do, at least. Once rejected I try to get past ever having had those feelings asap, rationalize them away, make myself accept it's not gonna ever happen, stop being stupid and deal with it, and move the fuck on.
I've noticed that guys seem to think that if I'm crushing on them I'll just keep crushing on them forever and they can just not say anything and sort of leave the situation on "pause", and try to return later like hey I'm available now you liked me right, but it doesn't work like that.
So, after two embarassingly public and very straightforward confessions, the ball was in Marco's court, and apparently he didn't reciprocate during his six weeks in Mewni but instead went back to Jackie. That's gotta sting, it's a bit humiliating, and any girl would interpret it as a clear rejection. So it's only natural that Star does her best to move on.