He got the house to where it needed to go, but by that time the house felt empty of Ellie, because she had transferred over to Russel in Karl's mind. The whole point of the movie is that Karl had to learn when to stop dwelling in the past and start a new adventure.
I got that, but still, having the house tied to them while they were walking around on the ground got a bit tedious after a while.
Oh, and if I ever hear people talking about physics and Pixar in the same conversation again, I think I'm going to pop a vessel. o_O I can't think of a better way to completely ruin a great movie.
I just think cartoon physics should be consistent with itself, e.g. the house shouldn't float more readily and be easier to push around with fewer balloons than it was previously with more balloons.