While not explicitly stated, Carlos must have passed away fairly recently in the Descendants universe. Which explains why Jane is not at the wedding; she is grieving and needs time to herself. Jane and Carlos were a couple, so she took the loss a lot more hard than the VKs. Mal, Evie and Jay had things to distract them. Mal was busy wedding planning and learning the duties of the queen. Evie has her fashion brand Evie's4Hearts in popular demand. Jay is on a gap year, exploring Auradon with Gil.