My own personal speculation is that the 4G radio is making development of a stable GB update for Thunderbolt a bit more difficult than a lot of other phones out there. This is fairly new technology...
I was optimistic when GB leaked a few weeks ago, but after reading how buggy it was and then the bricking issues (possibly tied to the radio), I realized there was probably still work to do.
I'm not in any rush...my TB is running juuuust fine as is with the BAMF remix 1.7
Frankly, I'd rather they get it right and make me wait an extra month, than push an update that's full of bugs and causes people's phones to reboot all the time...oh wait...