I've never forgotten a line that Hank Worden's cowpuncher character says about halfway through Red River: "I don't like it when things go too good and I don't like it when things go too bad....I like 'em in between."
Worden was talking about driving a huge cattle herd to market across rugged country, but most moviegoers feel the same way. They don't like films that are unrealistically happy or silly or dopey, and they don't films that seem oppressively glum and downbeat.
I can't think of a recent "too happy" film that qualifies, but the reason for Biutful's 71% Rotten Tomatoes rating, it seems clear, is that a certain percentage of critics are saying, "It's obviously very well made and Javier Bardem is great, but it's just too gloomy, dammit, and I wont have it...I want a few more rays of sunshine in my dramas, thank you."
What they're saying, in effect, is one of two things about their own life experience. Either they don't believe that bad luck comes in streaks for some people, particularly those who lack sufficient funds or cushions, or they know that this happens but they just don't want to go there. I myself feel that the downish stuff in Biutiful feels a bit much toward the end, but I know it can get that way (i.e., I've been close to this kind of thing in my own past) so I went with it.