Angel (TV): Nicholas Brendon's manager has released an official statement about recent rumours that Xander would move over to the spin-off: "Nick will absolutely not be going over to 'Angel'. The possibility has never been entertained, nor would Nick be interested if it had been mentioned"
Or the curse might simply not exist anymore. He fulfilled it when he had sex with Buffy, and then Willow restored the soul. His only resurgence into Angelus was because his soul was literally intentionally removed the second time.