Classical music and its performance is generally more 'formal', out of my lack of a better description, and tends to end up being taken more seriously, while popular music is generally more informal and could be described as 'casual'. Thus, they are going to end up having rather different atmospheres at performances, with different audiences with different expectations of the act.
Also, helps that at a popular music concert, the audience is going to be very loud throughout, covering up some imperfections, while the audience at a classical concert are bound to be mostly silent. (With the exception of the occasional cell phone ring, of course)