It's not only the problem of the graphics...
In my eyes, UO does have the best content of all the MMOs currently on the market. Having tried many other MMOs, I've been very quickly bored with them (really a matter of weeks). UO : I'm still there after about 8 years and still not bored.
But how can you explain this on a retail box ? It's simple : sadly, you can't.
On a retail box, you can put shiny concept art, epic blabla and pictures of the game (!)
In stores, on the extremely reduced PC games sections remaining, you still could try to sell UO :
But then, here comes the second problem of UO : try putting a box with pics of 2D or SA followed by the mention "Monthly fees 12.99$"... you'll sell nothing. When there's games like Guild Wars on the same shelf, with way not the same content of UO but still nice to play, with good graphics, populated and most important : free to play.
And putting a box on shelves doesn't automatically equal sales. You'll also need reviews of gaming magazines : people being informed that "there's a great 12 years old game making a brilliant come back and being for sale at your favorite store since september the 8th". Else, the majority will only ask himself "Why isn't this game in the 2$ bargain bin ?"
I think we really need to accept the fact that UO will never hit the stores again.
It still is a really good game, but it forgot to follow the evolution of the gaming industry these last years (of course keeping the same content and liberty that makes it one of the best games around). And it is just to late to magically reach the actual status of the market with the SA client or expansion.
We can really be happy that the game is still here, with a very low population (but still enough people to play with), and only count on word of mouth to bring new players in.
And of course trying to keep the existing ones : consolidating the player base !