User blog comment:Xilinoc/Ch. 589 - The Shooting Star Project: The Old and New Trust/@comment-6657474-20140717042206/@comment-6657474-20140722001954

Certainly possible, but considering that the Wandenreich's war is against the Souls Society itself, and that it was stated that after their defeat 1000 years ago, they deliberately fled to the Soul Society because it was lightly guarded at the time and, more importantly, the reishi concentration in the Soul Society is much higher than where ther came from (presumably the human world). It was the increased reishi in the Soul Society that enabled them to construct their base right under the Gotei 13's noses and likely a major contributing factor to their insane power levels, even among the Soldat. If they've spent most of their time training to fight the Gotei 13 WITHIN the reishi rich Soul Society, then it would likely put them at a major disadvantage to invade the Human World, where the concentration of reishi is much smaller. Not to mention they don't seem to have any quarrel with humans.