Two reasons why current staff is enough but doesn't seem like enough:
One, as many say, is due to inactive BAT members. But rather than focus on who is inactive, focus on how active the BAT are as a whole. Some BAT members really do not mod a lot (and I mean like three times a month or something) but I tend to disregard them as an accurate representation of the "BAT" identity.
The second reason is that people are getting lazy with their maps. What is the ratio, I wonder, of BAT posts to BAT bubble/ranks. In a perfect world, it would be 1:1. BATs are considered experts in the field of modding and their opinions are considered valuable. But that's no reason to just do a half-assed job, sit back, and wait until one of the BAT comes along and lists all the problems/improvements with the map so you can go an' fix 'em without having to look too hard. Yes, the kudosu system has done a great job of getting everyone to pitch in and help out, but I just want to restate a very important point (this is gonna enter someone's sig, probably)
The rate at which maps are ranked is not a reflection of the efficacy of the BAT, it is a reflection of the efficacy of the community as a whole.