The number of users in IRC channels tends to be quite high; the popover typically only has room to show a small proportion of these. This reduces the chance of the user you are looking for being visible, so you can simple click on them (or see them), rather than having to search or scroll. It also makes it harder to find a user by scrolling, particularly because it increases the chance of over or undershoot.

Ideally the popover would scale to fit the number of users in the list. If that's not possible I'd argue for making it taller. (In bug 738518 it was argued that list looks bad when there aren't many users, but this isn't the typical case for IRC in my experience.)