Why did the Black Butler anime diverge from the manga?
At the time the first season of the anime was developed, there was not enough published manga content to fill a season. 

The first episode of the Black Butler anime aired on Oct 2 2008. (IMDB) At that time, the last completed arc was the Curry arc. The first chapter of the Circus arc was not published until August 18, 2008. (Kuroshitsuji Wikia)