For a myriad of reasons:
The owners were trying to be an authentic Mexican team in MLS, Wanted all ethnic players, supposedly had to speak Spanish in training, treated non-Mexican coaches differently (there was a lawsuit by two coaches, maybe Calichman and another), didn't have good outreach/marketing in LA, paid rent to Galaxy to use stadium and couldn't balance books with low attendance, never had full support from owner - definitely his second team when compared to Chivas down under, and the players the mothership sent to the club always underperformed - many thought because they felt it a step down in MLS and didn't exert themselves.
The one thing they did have going for them was their academy was top notch. It became an orphan after Chivas USA collapsed and unfortunately LAFC has no interest in assimilating it - they want to start fresh.
I'm sure there's a lot more I'm forgetting about that fiasco....