Board Thread:General Discussion/@comment-24970687-20140531231739/@comment-5398287-20140601000530

I think it has to due with how dangerous it is simply to be near Zanka no Tachi. Remember, when Yamamoto used it against Royd, the latter only survived simply being in his presence because of Blut Vene - he even admitted he would've been ashes without it. Now, at that time, all of the other Shinigami, with the exception of Kenpachi, were spread out around the Seireitei, so they weren't near enough to be majorly affected by the heat beyond the little things we saw like their skin drying up and stuff (not to mention Kenpachi's so damn durable he could withstand it at a somewhat close range anyway). But in Karakura Town, all of the Shinigami were in a relatively close range to Yamamoto. Now, I remember perfectly well that Yamamoto was willing to sacrifice them with Ennetsu Jigoku, what with his speech about what the Gotei 13 is made for; however, Ichigo was also well within range of his Bankai, and had Yamamoto activated it without warning, poor Ichigo might've been incinerated, with the other option being Yamamoto warning him about it and Aizen getting the hell out of dodge.

tl;dr it was too dangerous at the time.

Either that or I'm reading too deeply into Kubo simply not wanting to reveal his Bankai at the time.