It might not be a complete answer, but wasn't it impossible for Spike to leave the Wolfram & Hart building until he materialized in corporeal form? During that time, he could not pick up a telephone to call anyone so only the W&H staff knew about him. By the time we get to "Damage", his presence is known but no one from Sunnydale (other than Andrew) makes any kind of effort to meet him. Maybe the writers were just tired of Spuffy.
Well, we know WHY Spike was on Angel. He was there to help 'save' it -- to spice things up on a dying (dead) series. After season 4, they knew they had to do something big to keep the show on the air, so they revived Spike. They had to contrive a situation where he stuck around. Rushing off to be with Buffy would not make that happen.
My own head canon/musings.
One thought I had that would be consistent with who Spike was: Spike was somehow under the impression that Buffy knew he was alive and never reached out. Since he wasn't sure of her, he didn't go running to look for her. He might have assumed (or been told) that Andrew had told her he was alive. I could easily see him staying away if he felt that she's 'moved on' enough not even to contact him.
I could also believe that after months to process his soul and his actions before getting it, he truly came to believe he wasn't good enough to deserve her. After all, coming back to Sunnydale right after the soul means he didn't really think about coming back, his mind was a jangled mess and he just did what his instincts told him. Being in Sunnydale during a demonically altered (even more than usual) time, didn't give him time to process much of anything either. Also, he might have realized he was needed even if he thought staying wasn't in Buffy's personal best interest. After coming back from the dead incorporeal with nothing but time to think about his existence, I would not be surprised if he became afraid/hesitant to go searching for his Buffy dream.
Finally, he's in L.A. with Mr. "I'm doing what's good for Buffy and staying away" -- I cannot believe
that wouldn't rub off on him just a little bit... At the same time as I believe his feelings of guilt over his past would increase once he was out of the emotion-warping ever-burning hell hole that was Sunnydale.
I always had a theory that the Hellmouth was bad for ALL relationships.