In short, it doesn't. The length of a single player campaign that is full cost, has no other modes, and no replay ability is an important part to think about. Mainly because you deserve to know "Hey this game is great, but short. So if money is tight and you want a lot of bang for your buck then consider that before getting it".
But since this game is apparently not really good it's just another reason for people to complain about it.
Really this is probably what makes games so much harder to review than movies. Movies cost the same to see no matter what (unless you have to watch them in 3D). So it's never about length, it's about quality. Sure some movies are longer than others, but no one ever associates a 3 hr movie being a better value than a 1 1/2 hr movie.
This is what made Dan's review of that Toad game so odd but fair. The reality is that most people don't read a full review. They'll look at the numbers and say 9.0 ok great I'll buy it. So the only way to express the problem with the length of a game, in a way that the general audience can see, is in the final score.