I know that the case with MtGox is old enough to be care of and now many exchanges are running a two-factor authentication, but still there are lots of scams and fake sites.
The higher the volume means more people are buying and selling on that site.Gives more credibility.And yes they fluctuate frequently but they have been consistently at the top. I think more people use bithumb because they have one of the lowest transaction fees (.15%)