I suppose it depends on the type of conference, but I suspect it's a cultural thing. The scientific community is filled with people who are collaborating, yet simultaneously competing. It's also practically a required trait for a scientist to be skeptical of any and all claims and data that are presented. So people challenge things left and right, partly because that's what they're supposed to do, and partly because they may want to drag down a competitor's work (while perhaps simultaneously bringing up their own). If praise is given, it's usually in private.
Actually, this may apply even outside of the scientific community. When is the last time you heard someone giving someone else a random compliment? Compare that to the last time (or the number of times) you've heard people criticizing or saying negative things to others. In my part of the world, at least, it seems that people have little reserve about saying something negative, yet when it comes to giving praise, they're much more shy about it... even if they were moved or impressed, they'll keep it to themselves or their friends, and won't go out of their way to give their praise to the person who earned it. Having noted that, I do my best to give positive remarks and compliments whenever the thought comes to my mind. It's a small effort to make the world a better, brighter place