It could be that the First knew that the Slayer line would be extended, making the balance between good and evil more solidified - and that a resurrected Slayer and a souled vampire would help bring this about. In S3, Buffy had already been resurrected once, and a souled vampire (Angel) was in Sunnydale. So the First made its move in "Amends".
But, Angel left Sunnydale and the First believed it had nothing to worry about. Until S6, when Buffy came from the dead again . . . and Spike regained his soul at the end of the season. And returned to Sunnydale before S7.