Because in the space of three months we have had two seasons of TNG, one of ENT and a new movie. Even in the late nineties fans weren't asked to shell out for this many hours of Trek all at once.
A lot of it is because of the new movie. It is standard when new movies are released in popular franchises to really pump product out related to it. The James Bond movies were re-issued last year prior to Skyfall's release, there was a ton of Batman stuff last summer before The Dark Knight Rises was released. Even before Trek XI's release in 2009 all three seasons of TOS and all ten Prime movies got their first blu-ray release.