User blog comment:Mixoris/Gate/@comment-3134619-20120227215328/@comment-3134619-20120227223152

Well, since there weren't any Hollows in the Soul Society arc, it didn't show up there, and I'm not sure if the gate to Hell would show up in Hueco Mundo or not, but, really, it's probably because that, after Kubo introduced the concept and reinforced it a bit, he switched to focusing on other plot devices, trusting that people would remember how it worked. Or, it could have even slipped his mind/fell out of importance in comparison to the other plot devices... Really, the gate to Hell only opens when a Hollow had been a particularly wicked human in it's previous life, so it won't open for all of them.