@Esteban  
Or they just thought Discord was a better fit for the character he’s supposed to be parodying instead of Starlight or any other Main character in the show even Celestia.  
Celestia’s more of a main cast member than Discord and she’s nowhere here, doesn’t make her less of a main cast member.  
But whatever…

Yup, this is the main cast of MLP. Mane6 + Spike and Discord. Notice that there is no Glimmer in it because she is just an extra OC that overstayed her welcome thanks to some few poor show runners.