Board Thread:General Discussion/@comment-24382631-20140110234616/@comment-1268838-20140602215126

Even after his initial Hougyoku transformation, Aizen's spiritual pressure had ascended to such a height that beings of lesser power could not even comprehend it, unless he voluntarily reduced his power. The reason that Ichigo even stood a chance against him was because he had taken it a step further and had even greater spiritual pressure, taking it to an extent that not even Aizen could comprehend it. And even then, Aizen survived, being essentially immortal.

As powerful as Yamamoto was, we have never really heard anything to suggest that his power quite approached that level. And of course, as we know, battles between spiritual beings are based entirely on Reiatsu. It is true that Ryujin Jakka was a threat to Aizen, Zanka no Tachi undoubtedly more so, but that might not have been applicable after Aizen started to transform.