I think the 2004 dvd's sold well because dvd was a big deal to the masses. And the idea of having special features for Star Wars was cool. (I wasn't aware at the time there were plenty of existing extras on laserdisc and video, I didn't even realize what laserdisc was) I don't think Blu-ray is as significant for the average person. The average guy was blown away with dvd. Now the average person is saying "that's nice" or "huh, when was dvd not good enough?" Remember, the masses did not have laserdisc so the leap from vhs to dvd is just so huge.
-scene selection
-no rewinding
-much greater resolution and clarity than vhs
-dvd's can be played on computer drives and portable machines
-dvd's last longer than tapes, more durable too
DVD's appealed to just about everyone over vhs, hence full screen releases.
The casual fans are not going to care about upgrading from dvd to blu-ray. The die hards do. Guess which group also cares about having the original versions? Honestly, the novelty of dvd has worn off for me. I don't buy nearly as many movies on dvd, special features are no longer as important to me. You can find a lot of stuff on youtube now anyways. That site didn't even exist in '04. I've passed on upgrading to many newer dvd versions.
I think a lot of people are like that too. How else do you explain most new dvd releases that seem to be lackluster in extras? I mean a popular family film like Enchanted is just one disc with few extras.
I know about the technical difference between blu-ray HD and regular dvd. And I don't really care. I'm interested in the higher storage capacity of blu-ray, that's about it. Oh and if any movies I really care about get a vastly improved transfer (besides the usual higher resolution) or some really great bonus content (archival content like deleted scenes would probably appeal to me most).
Star Wars could fit that category. Studio Ghibli (because they're awesome) and Buffy the Vampire Slayer (less compression on fewer discs?) is another possibility.
I also think more people read internet reviews so if something is wrong with a video release, more will hear about it.

In my opinion, just releasing the latest version of the special editions on blu-ray will NOT sell as well as the 2004 set did.
(Plus think of all the complaints from people who fell in love with the '04 versions when Lucas does something even worse than Hayden. "I miss Han and Greedo shooting each other, that brotherly hug is so lame! And did they really need another musical dance number added when Luke meets the Emperor?")