Its because of the art style.When you go for a more realistic artstyle, the game ages much more quickly, and a hd remaster can only do so muc, while artstyles like WW ages really well and thus looks better when remastered

The reason why TPHD doesn't 'look as good' as Wind Waker's is really a matter of art style. Wind Waker is simplistic, which made it easier to give the HD treatment and it really shows. TP on the other hand is more realistic and far more detailed, thus everything in the game was very refined, down to every little nook and cranny in even the smallest things, like symbols on the walls, textures on characters' clothes, and the extra amount of detail on midna's head piece. You have to look closely at everything in comparison to the Wii version, and it shows quite a bit.

Considering how much there was to touch up on TP, it would take so much more work than WW.

This is basically exactly what I'd expect from an HD remaster of a Cube game.

I definitely was disappointed when I saw how the game looked. The Wii U tech demo at e3 looked amazing because they remade the textures, lighting, shading and everything else. That would probably take years to do though, as well as be very expensive. The updated textures do look much sharper though. I just wish I could buy the wolf Link amiibo separately!

Twilight Princess deserved a full remake on the NX, or at least a good remaster with added make up, new lightning effects, and details, like WWHD.

It looks just like a lot of the other 6th gen to 7th gen HD ports. Windwaker only looks as good as it does because it was cel-shaded. We were never going to get the tech demo graphics, because that would have required the game to be completely remade.

Twilight Princess HD doesn't look as good as Wind Waker HD because Twilight Princess doesn't look as good as Wind Waker. It's pretty simple really.

Can't say better than this

This and the fact that making deeper changes would mean major modifications to the engine or even a totally new engine for the game, at which point they couldn't reuse many of the assets. If the remake would have been planned to push the capabilities of the Wii U, they would have needed to throw most assets away and basically restart from scratch. That's also the reason why all those remakes out there are from last gen consoles, older games are just not worth the effort anymore since nothing can be reused here.

Agreed. Original TP = ugly, thus more likely for this version to not be as nice as we would expect. 

Original WW = very nice and somewhat timeless looking --> WWHD = gorgeous (imo)

This is why I am realy looking forward to Zelda U's aesthetics; it has a wonderful sense of style that will not age for a very very long time, if ever