Nothing more to it than the simple fact that the team working on the unlock has not yet found an exploitable bug reliable enough to build an unlock on for the newer basebands. There's no guarantee that they ever will -- nothing says specifically that an exploit does exist in these basebands that will allow for an unlock.
Every time that Apple updates the baseband it gets harder and harder to find a workable exploit. It's worth mention that ultimately over the long term, Apple is probably going to win -- not that anything can ever be 'bulletproof', but it's definitely possible that as they play Cat and Mouse with the dev-team and unlockers, that they will end up finding ways to really stop the unlockers in their tracks for longer and longer periods of time (as they have now). The proof is in the pudding presently -- there are basebands that have been out for over a year that do not yet have soft-unlocks.
That said, most (but I can appreciate not all) people who ended up needing 06.15.00 only ended up needing it because they updated when they shouldn't have or didn't follow safety instructions to ensure their basebands were at a low enough level to unlock. I only have a small amount of pity for people who accidentally upgraded themselves -- the dev team, the mods here, and all of the people working on the unlocks remind everyone every time we're asked that you should never upgrade iOS without doing homework first. That said, I can appreciate that there are some people out there who bought the 3GS new or used with a version of iOS / baseband on it that was not unlockable, and there was nothing the end user could do about it.
I still anticipate that we will see an unlock in the not too distant future. It still makes good sense for the unlockers to hold off until 5.1 is released, as it is expected that a new baseband will exist in the 5.1 update for all devices -- and if they hold off until then, they can roll the dice that the exploit will still exist in the 5.1 baseband, where if they publish it for the 5.0.1 baseband, it is guaranteed to be fixed. Worst case, if 5.1's baseband blocks the unlock and they can't repurpose it for the newer baseband, they'll just release it then.
I definitely understand that it sucks to have a phone that you can't use, and I also understand that the iPad baseband solution really isn't a good solution for most people -- but unfortunately that's really just the way things are now. There's nothing to be done other than wait.