Did you even watch the show?
Marco was in love with Jackie for years, he discarded it being a "teen love" in front of Janna, Jackie, Star, Ponyhead, Starfan 13 and the Cube of Truth or Punishment, which was true, the cube itself stopped from destroying their surroundings and being a menace to congratulate his autoanalysis, if that was not enough, a curse, Naysaya, also activated with a VERY SPECIFIC situation, the afflicted one needed to talk with his/her true love, so the curse revealed his secrets. Janna read this from a book and not only the Naysaya confirmed it but the writers made an effort to make it clear that anything Janna does, says, read or knows about anything occult-related is BASED AS FUCK.
So, having her return and say "hey meet my girl friend here, let me hold her hands to imply we are a couple and tell you Marco that Star is better than me so morons believe it" was TOO important, because if anything was affecting Marco's behaviour was the Curse. That shit changed his life, to worst, it ruined his whole development as a the deuteragonist that got the girl he liked, the trainning he desired, the adventures he needed and the maturity he looked to achieve, Marco was free to leave the show after Battle for Mewni and show up to help Star defeat Meteora only, but the bastards in charge of the show wanted to drag him around and force him in a relationship with Star, someone he NEVER LOVED, and lie to all of us, watchers, saying "it was love at first sight"
Is it clear now why was turning Jackie a pinpoint to force Starco even more? If Jackie was free when they returned back and Marco was free from the curse, there was no reason for him to not feel like begging pardon to Jackie, explaining he was acting like an obsessed douche because of a magical affliction and try to win her love back again. Jackie had found and developed strong feelings towards Marco during their time together, that is a fact.