Didn't the Powers say at one point that they couldn't, or maybe it was wouldn't, interfere directly in the day to day affairs of mankind? That was the whole purpose of beings like Whistler, Doyle, Cordy, Angel, and, probably to a lesser extent, the Slayer line. The Powers informed their champions, either through visions or Slayer dreams, of what was about to happen, but the champions were never forced to act against their own free wills. With Cordy gone, Angel didn't have any connection with the Powers, so, in theory, they would have either found or been in search of, a new champion
Then again, maybe the containment of the First Evil and the sudden influx of Slayers after Sunnydale tilted the scales so far in their favor that they just decided to take a break. Then the Senior Partners acted quickly and struck a bargain with Angel before the Powers knew just what was going on.