Why was a prequel made instead of an adaptation for Ga-Rei? I enjoyed the Ga-Rei Zero anime and the Ga-Rei manga (now finished) quite a bit. 

With the anime industry seemingly grasping at straws for manga to adapt, why was a prequel animated instead of the actual source material?
Ga-Rei: Zero was released in 2008-10-05. If the marketing and industry people want to work on a 6 month lead time, this puts decision time at 2008-4-01. At that time, the most recent tankōbon release was volume 5 back in 2007-10-26. Also, the mangaka took longer to publish material as the series progressed. See below:

1   05/26/06    days
2   07/26/06    61
3   11/25/06    122
4   04/26/07    152
5   10/26/07    183
6   04/26/08    183


Maybe the manga was popular and they wanted to get that target audience but didn't have a lot confidence in how the current story arc which was still being written would adapt to a 13 episode series they decided to hijack the plot and audience and make their own version of it. It's just speculation though. I did like this series. The first episode had me totally lost after having read the manga though :/