There's also the conservation-of-space factor to consider. The entire lot were something like 60 discs for all of TNG and 30-something for TOS I think. A lot of shelf space needed. The 'complete season' boxsets took only a fraction the same shelf space.
On the other hand, the spine art on those DVDs was
impressive. I never owned them, but I did know someone who did and the TNG discs in particular looked stunning on the shelf, with their montage featuring the Enterprise, a couple of Borg ships, and a Romulan warbird. It looked amazing.