It's a superior format to DVD.
From what I've seen, not really.
But the main reason is this: I live on a fixed income and after ten years, I still have not completed upgrading my VHS collection to DVD, and I'm not about to start all over and spend ANOTHER decade doing the same to Blue Ray. What little money I have to spend on entertainment is better spent on other things.
It's literally that simple.
I certainly get the fixed income thing. It's not clear from your post, but if you haven't already collected TNG on DVD, then you aren't "double dipping". So why not skip collecting the series on DVDs and start collecting it on Blu-ray to start with?
I've long since collected all the Star Trek series and movies on DVD, so it would be double-dipping.
No, DVD is good enough.