Similar to 3DS equivalents, Wii U demos come included with a play limit - meaning that after you've played a demo a certain number of times you can no longer access it, leaving it unavailable for further use.
Eurogamer.net questioned Nintendo on why limits vary with each demo title, and why such a limit exists in the first place. A representative replied:
Each publisher has the opportunity to decide how long demos will stay on the eShop, and how many times consumers can play them before they expire.
Once the demo has passed its limit and you try to enter it again, you will be asked whether you would like to view the full game title over on the Nintendo eShop - if the full game of that demo is available, you have the option to purchase it. What's your view on the play limits? As usual, let us know.