They should've released Enterprise in late-2009/early-2010 when it could've capitalized on the success of Star Trek 2009. Releasing it now means its competing with TNG for home video dollars.
Bear in mind that around that time we got TOS and the films on Blu-ray too. Maybe TPTB didn't want to flood the market with too much Trek product. Also the DVD's only came out in 2005 so maybe they felt it was a bit too soon as well. Releasing it now makes perfect sense. General interest in Trek will be high thanks to Star Trek Into Darkness.
As for DS9/VOY if they stick to the schedule they seem to have with TNG (a season every six months or so) the DS9 Blu-rays will be out sometime around 2015-2018 and VOY will be out around 2019-2022. This assumes they carry on after TNG with the same system they have in place now.