Canna did not immediately accept Purple Diamond's invitation.
The words that the other party blurted out were too shocking; betraying the kingdom sounded very unreliable. Even Canna, who generally stayed out of worldly affairs, found it unfeasible.
Moreover, no matter what, she was still a Card Magical Girl with a duty to fulfill. Just by sincerely going to the Material Realm and killing a few more high-ranked Remnant Beasts or fighting the rebels in the interrealm to earn some merits, theoretically, there was still a possibility of retaining her identity. Although this possibility was not great, it was enough for her to refuse to join the other party's madness.
