Some senior members are considered as specialists on their field. If you see the 'grizzled veteran' (Raskolnikov) you know he'll tell you something valuable. Chris Mood is always there when you talk about Jazz (be it technique or theory) After a while, people will see you as a serious guy even though you aren't a moderator.
Then, why some of them are moderators and some aren't? Well, be in the right place at the right time. Some people around here could wipe the floor with my face in a technique contest,and I'm not an expert in every style, but I was the one that applied... Be patient...