You're comparing apples and oranges. A metal concert isn't an exclusive club; anyone can buy a ticket and go. And yes, most salarymen in Tokyo wear office suits every day, so since it was on a Monday, they just didn't bother bringing a change of clothes. I see men in such suits all the time on weekdays doing whatever, because those are their work clothes.

It's unfortunate that the rock and metal crowd is mostly middle-aged people these days; the young people seem to be listening to rather bland pop music, idol music, etc.