I think the technical answer is that the shows were airing on different stations and SMG didn't really have time to work on a crossover episode. The Cangel fan answer is "cuz he didn't like Buffy anymore".
My answer? Because I don't think Buffy would have gone along with it. Seriously...it would be like "we want you to sleep with the guy you really love just cuz we need him soulless." How would you feel? And I don't think that Buffy would want to be part of anything that brought Angelus back into the world. Plus Buffy was really busy dealing with her own major apocalypse in Sunnydale at that time (the spin off where they want Angel to lose his soul, is in season 4 of Angel which corresponds with season 7 of Buffy). Until I think of some more reasons, that's what I'm gonna stick with :)
My answer? Because I don't think Buffy would have gone along with it. Seriously...it would be like "we want you to sleep with the guy you really love just cuz we need him soulless." How would you feel? And I don't think that Buffy would want to be part of anything that brought Angelus back into the world. Plus Buffy was really busy dealing with her own major apocalypse in Sunnydale at that time (the spin off where they want Angel to lose his soul, is in season 4 of Angel which corresponds with season 7 of Buffy). Until I think of some more reasons, that's what I'm gonna stick with :)