Here's why:
* Aizen's imprisonment: Aizen was sealed away in the Muken, a special prison designed for him, after his defeat by Ichigo and his friends.
* Marmalade's appearance: Marmalade appears during the "Lost Agent" arc of the Bleach manga, which takes place *after* Aizen is imprisoned.
* Aizen's escape: Aizen later breaks out of Muken through a complex scheme, but this is separate from Marmalade's appearance.
So, while Aizen is not dead, he is imprisoned for the majority of the time Marmalade is active.