[nobody7290]

I think nobody knows a real answer, you can only speculate.

It could be because:
RIM simply does not want to leak unfinished builds.

RIMs development team is busy developing the playbook - sounds unlikely, because I think the playbook dev team is a different team then the blackberry team, but since RIM will move everything to QNX OS, maybe the BB dev team needs training to be able to develop for QNX and therefore there is less time for Development ?

RIM focuses on improvement of current devices then on old ones which already run stable. Why fix something which works ? Why add features to devices which do not have the memory or CPU for new things ? For me this is the most likely reason.

I tried very hard for one year to use OS6 on my 9700, but finally gave up and upgraded to OS5. IMO it was a wrong decision to offer an interim OS for the 9700 which not only has bugs and is bloated because of the mobile safari, but also has less features then OS5.