User blog comment:Xilinoc/Ch. 591 - Marching Out the ZOMBIES 2/@comment-24509822-20140731013131/@comment-24384892-20140802213925

What if the reason why Yhwach didn't have Giselle zombify Aizen is because Giselle can't, due to Aizen's immense spiritual power? Aizen mentioned that an opponent with greater spiritual power can negate another opponent's abilities with lower spiritual power. An example would be when Aizen rendered Soifon's shikai useless due to him having much higher spiritual power than her.

If that's the case, then Giselle can't zombify Aizen even if she/he wanted to. His monstrous spiritual power would just negate her zombifying power.