I'm not sure what's going on with your tree, but here are a few thoughts. I noticed that the leaves in addition to yellowing look chewed on; perhaps it has some kind of insect infestation? It wouldn't take many to harm a small seedling like that. You could also check the tree tube for mouse nests. They like the warmer conditions inside the tube and will set up shop there. They then eat at the base of the seedling and will eventually kill it. It could be location too. Even though another oak close by is doing well, there can be differences over that short of a distance (the fancy term is "microclimate"). The spot where this tree is may be, for whatever reason, not a great location for it. Finally, there's genetics. A young forest will have thousands of seedlings per acre, but by the time they grow up there may only be a hundred or so. Less sturdy ones don't make it and die off on their own. Even with the care and protection that it sounds like you're giving this tree, you won't get 100% survival.