The difference is there, but minuscule, barely noticeable, and only registered if you take a hard look at a side by side comparison. Tech Demo graphics would be more expensive, yes, but not as expensive as making a full new game, would further justify a price over $30 and would be fitting since everyone other remake has gotten that treatment.
The fact that fans are able to consistently make more noticeable and meaningful graphical differences with much fewer resources and training is just disgraceful. More annoying is Nintendo showing us what their system is capable of to begin with, and then performing below their ability in its later life. No matter which way you try to look at it, it just comes across as lazy.
Living up to the tech demo shouldn't even be a discussion anymore. That was shown to us at the beginning of the Wii U's life cycle. We're reaching the end, and by now we should be talking about surpassing the tech demo.