Oh, THAT assumption again. I should have thought of that. Herewith an attempted perspective shift.
"First in, first out" doesn't have, and can't have, any meaning to us. Think of it this way: we become interested in a site as soon as it's completed -- NOT at all when it's submitted. After all, a site may be submitted before it's completed, or afterwards, or never! (And sites that are NEVER submitted are as precious to us as sites that have been submitted a thousand times ... this week.) So submittal date logically has nothing to do with anything.
All that matters, all that can matter, to us is that the site is up and functioning. Of course, we can't do anything about it until we find out about it. And that date is almost (but not quite) as irrelevant as submittal date -- at least we can't find out about the site before it exists!
So we don't worry about ANY dates. A site is up RAT NOW (and eligible for listing), or it's not (and it's not).
Now, all that is true and obvious to any experienced editor. It is also obvious to any editor (experienced or not) that there are many good reasons to review sites out of the submittal-date-based order. (That is, it is significantly WORSE to use that order than it would be to use some completely random order.)
In addition, note that there isn't a single queue; each category has its own queue. Each queue has its own set of editors, with their own personal priorities between queues. And an editor in one category may be able to do nothing more than move a site to another category queue, to start waiting again (on a different set of editors). FIFO couldn't have meaning in such a context even if we wanted it to.
And finally, note that our freedom to handle sites in any order is an extremely powerful weapon against the spammers who provide most of our site submittals. If for no other reason, by now we would probably have been forced to introduce some form of indeterminacy to foil systematic probing attacks by wannabe-very-high-volume-automated-spammers.
Which comes around to the other answer. Your submittal is waiting because most of editors' time is wasted by spam submittals. Were it not for those, with our current rate of volunteerism, we'd have far more than adequate manpower to handle all legitimate submittals.
