So UU banned the original BLs, then kept going. You don't unban something just to balance the metagame out, because it is still broken. For example, we didnt get Raikou to come back just because Crobat was getting out of hand. We didn't get Crobat to comeback because Yanmega and Shaymin were out of hand. So, we can't just drop some Ubers to "balance out our suspects". That's just not how the process works, and what you are suggesting is completely illogical and I don't need to tell you that it won't work.
'Course not. If that was the only reason for something to bring down something from Ubers and try it again, there would basically be no case for it, and things would be left as they are. However, there are reasonings that some of the things that are currently banned may no longer be so broken as compared to the time of their banning, such as Hypnosis's accuracy having decreased, and things of that nature. It is due to this that it is felt that certain Pokemon should be retested.
Beyond that though, retesting has another potential benefit. Let's say, for example, that the changes that have occurred since it's banning have indeed been enough to no longer make Yanma quite so broken, and it's LC OU material.
Now, for the purposes of example (I'm of course not saying this is true at all or anything; the following is a what-if to get my point across, and nothing more, and I'm not trying to say this particular scenario is in any way feasible or anything but use it as a means to convey my point), let's shift the focus
to another suspect, but one in the opposite direction: Misdreavus. Right now, NP Missy is looking quite broken. However, let's now say that Yanma and oh say Murkrow are thrown downwards before any decisions on Missy are reached, and both are now looking to fit in just fine into the LC OU metagame without any real problems. In addition to this though, the changes Yanma and Murkrow brought to the metagame when they were re-introduced, along with their own merits, seems to have been enough to balance out Missy and it's no longer really giving anyone too much trouble. Thus, Missy, Murkrow, and Yanma all get to stick around.
However, going with the same scenario, if we followed the method of "test the new suspects first, then drop Ubers down", Misdreavus would wind up being banned due to being broken in the current metagame. Yanma and Murkrow would still most like wind up being dropped down just fine in that scenario, but because Missy was banned prior to testing it in conjunction with Yanma and Murkrow, it's just thought of as being broken by all and the fact that if it were simply dropped down again is not known. Now, Misdreavus may of course be proposed to be moved down after this in the next test in consideration of how the Yanma and Murkrow tests went, but this would be done in a second test, which wouldn't have even been needed had it just been tested in conjunction with Yanma and Murkrow, the re-test candidates, in the first place, making such a process way more time-consuming than it needs to be.
Of course, whether one or two retest candidates may be able to have enough of an effect to create such a difference may not be the most common thing to occur, but it is possible, especially when it's a very close call on the suspect's status, with it really walking the line. Thus, just to make sure and also in the interest of test-efficiency, test-productivity, killing multiple birds with one stone, etc, dropping down the potential retest candidates first and going from there seems to be the preferable and more logical choice, IMO.
To sum this all up: A Pokemon is proposed to move down from Ubers when it is believed that there has been a significant enough change in the metagame since it's banning that it could potentially no longer be broken in standard play. When this is done, it is done to see whether that Pokemon is broken in the current metagame, at whatever stage or form it's at at the time. The effect of it balancing out any current potential suspects in the Standard metagame is just a
bonus to this should it occur, and not the main goal. It's just a bonus, which allows the process to go more smoothly and streamlined, cutting out the need for potential extra tests as compared to the other method, should it occur. The main focus though is still seeing if the Pokemon up for being retested are still broken or not; if they are still broken, oh well, send 'em back, but if not, that's great, especially if it works out that they end up keeping other things in line too. That's the reasoning for this method.